Concentra Biosciences, LLC, a Delaware limited liability company (“Parent” or “Purchaser”), is offering to purchase (the “Offer”) all outstanding shares of voting and non-voting common stock, par value $0.01 per share (“Shares”), of IGM Biosciences, Inc., a Delaware corporation (“IGM”), for: (i) $1.247 per Share in cash (the “Cash Amount”); and (ii) one non-transferable contractual contingent value right for each Share (each, a “CVR,” and each CVR together with the Cash Amount, the “Offer Price”), all upon the terms and subject to the conditions described in this Offer to Purchase (together with any amendments or supplements hereto, this “Offer to Purchase”) and in the related Letter of Transmittal (together with any amendments or supplements thereto, the “Letter of Transmittal”). Subject to the terms of the Merger Agreement (as defined below) and the CVR Agreement (as defined below), the Offer Price will be paid net of any applicable tax withholding and without interest.

The Offer is being made pursuant to the Agreement and Plan of Merger, dated as of July 1, 2025 (together with any amendments or supplements thereto, the “Merger Agreement”), among IGM, Parent and Concentra Merger Sub V, Inc., a Delaware corporation (“Merger Sub”) and a wholly owned subsidiary of Parent, pursuant to which, after the completion of the Offer and the satisfaction or waiver of certain conditions, Merger Sub will be merged with and into IGM, without a meeting or any further action of the IGM stockholders in accordance with Section 251(h) of the General Corporation Law of the State of Delaware (the “DGCL”), and IGM will be the surviving corporation and a wholly owned subsidiary of Parent (such corporation, the “Surviving Corporation” and such merger, the “Merger”). Accordingly, Merger Sub is considered a co-offeror in the Offer. The time at which the Merger becomes effective is referred to as the “Effective Time” and the date upon which the Merger becomes effective is the “Closing Date.” Upon the terms and subject to the satisfaction or waiver of the conditions of the Offer and the Merger Agreement, including the Minimum Tender Condition (as defined below), Purchaser will accept for payment (the date and time of such acceptance, the “Offer Closing Time”) and thereafter pay for all Shares validly tendered and not properly withdrawn pursuant to the Offer promptly after the Expiration Date.

Concurrently with the execution of the Merger Agreement, and as a condition and inducement to IGM’s willingness to enter into the Merger Agreement, Tang Capital Partners, LP, a Delaware limited partnership (“TCP” or “Guarantor”) and sole member of Parent, delivered to IGM a duly executed limited guaranty (the “Limited Guaranty”), dated as of the date of the Merger Agreement, in favor of IGM, in respect of certain obligations of Parent and Merger Sub under the Merger Agreement and the CVR Agreement. Certain obligations under the Limited Guaranty are subject to a cap of $78,000,000, which includes certain Enforcement Costs under the Merger Agreement and an amount equivalent to the CVR Proceeds (as defined below) under the CVR Agreement. Tang Capital Management, LLC, a Delaware limited liability company (“TCM”), is the sole manager of Parent and the general partner of TCP. Accordingly, TCP and TCM are considered co-offerors in the Offer. As co-offerors, TCP and TCM accept joint responsibility for the accuracy of the disclosures made in this Offer to Purchase.

Pursuant to the terms of the Merger Agreement, at the Effective Time, by virtue of the Merger and without any action on the part of the holders of Shares, each outstanding Share, other than Shares held in the treasury by IGM, held by any stockholders, or owned by any beneficial owners, of IGM who are entitled to and who properly exercise appraisal rights under Delaware law or owned by Parent, Merger Sub or any other subsidiary of Parent at the commencement of the Offer and is owned by Parent, Merger Sub or any other subsidiary of Parent immediately prior to the Effective Time, will be converted into the right to receive the Offer Price, without interest and subject to any applicable tax withholding. As of immediately prior to the Effective Time, the vesting for each option to purchase Shares from IGM (“Company Stock Options,” and each a “Company Stock Option”) and, as of immediately prior to the Offer Closing Time, the vesting for each restricted stock units granted by IGM (“Company Restricted Stock Units,” and each a “Company Restricted Stock Unit”), shall be accelerated and: (i) (A) each Company Stock Option that has an exercise price per share that is less than the Cash Amount (each, an “In-the-Money Option”) that is then outstanding will be cancelled and, in exchange therefor, the holder of such cancelled In-the-Money Option will be entitled to receive in consideration of the cancellation of such In-the-Money Option (1) an amount in cash without interest, less any applicable tax withholding, equal to the product obtained by multiplying (x) the excess of the Cash Amount over the exercise price per share of Shares underlying such In-the-Money Option by (y) the number of Shares underlying such In-the-Money Option as of immediately prior to the Effective Time, and (2) one CVR for each Share underlying such In-the-Money Option, and (B) each Company Stock Option that has an exercise price per share that is equal to or greater than the Cash Amount that is then outstanding will be cancelled for no consideration; and (ii) each Company Restricted Stock Unit that is then outstanding will be cancelled and, in exchange therefor, the holder of such cancelled Company Restricted Stock Unit will be entitled to receive in consideration of the cancellation of such Company Restricted Stock Unit (A) an amount in cash without interest, less any applicable tax withholding, equal to the Cash Amount, and (B) one CVR.

Prior to the Effective Time, IGM shall satisfy all of its notification requirements under the terms of each outstanding and unexercised Company Pre-Funded Warrant, and, at the Effective Time, each Company Pre-Funded Warrant shall be canceled and holders of such warrants will be entitled to receive: (1) an amount in cash without interest, less any applicable tax withholding, equal to the product obtained by multiplying (x) the excess of the Cash Amount over the exercise price per share of such Company Pre-Funded Warrant by (y) the number of Shares underlying such Company Pre-Funded Warrant as of immediately prior to the Effective Time; and (2) one CVR for each Share underlying such Company Pre-Funded Warrant.

As noted in the Summary Term Sheet, there is a risk that you may receive no payments under the CVRs. Therefore, in making a decision to tender your Shares, you should understand that if the CVR does not generate any payments, the only consideration that you would receive is the Cash Amount of $1.247 per Share that is being offered pursuant to the Offer. You should base your tender decision on the Cash Amount as it may be the only consideration you receive in the Offer. On July 15, 2025, the last full trading day prior to the date of this Offer to Purchase, the closing price of IGM’s common stock as reported on Nasdaq was $1.35 per Share.

On July 1, 2025, the IGM board of directors (the “IGM Board”) at a duly called and held meeting, duly and by unanimous vote: (i) determined that the terms of the Merger Agreement, the Offer and the other transactions contemplated by the Merger Agreement and the CVR Agreement (collectively, the “Transactions”) are advisable, fair to, and in the best interests of, IGM and IGM’s stockholders; (ii) approved and declared advisable the Merger Agreement and the Transactions; (iii) authorized and

approved the execution, delivery and performance by IGM of the Merger Agreement and the consummation by IGM of the Transactions; (iv) resolved to recommend that IGM’s stockholders accept the Offer and tender their shares of IGM Common Stock in the Offer; and (v) resolved that the Merger Agreement and the Merger shall be governed by and effected under Section 251(h) of the DGCL and the Merger will be consummated as soon as practicable following the Offer Closing Time.

The Offer is subject to various conditions. See “The Tender Offer—Section 9. Conditions of the Offer.” A summary of the principal terms of the Offer appears on pages 45 through 46 of this Offer to Purchase. You should read this entire document carefully before deciding whether to tender your Shares.

Neither the U.S. Securities and Exchange Commission nor any state securities commission has approved or disapproved of this transaction, passed upon the merits or fairness of such transaction or passed upon the adequacy or accuracy of the information contained in this document. Any representation to the contrary is a criminal offense.

Unless otherwise indicated in this Offer to Purchase or the context otherwise requires, all references in this Offer to Purchase to “we,” “our,” or “us” refer to Parent, Merger Sub, Guarantor or TCP as the context requires.

If you desire to tender all or any portion of your Shares to us pursuant to the Offer, you should either: (i) if you hold your Shares directly as the registered owner, complete and sign the Letter of Transmittal for the Offer, which is enclosed with this Offer to Purchase, in accordance with the instructions contained in the Letter of Transmittal, mail or deliver the Letter of Transmittal and any other required documents to Broadridge Corporate Issuer Solutions, LLC (the “Depositary and Paying Agent”), and deliver the certificates (if any) for your Shares to the Depositary and Paying Agent along with the Letter of Transmittal of this Offer to Purchase prior to the expiration of the Offer; or (ii) if you hold your Shares in “street name,” request that your broker, dealer, commercial bank, trust company or other nominee effect the transaction for you. If you hold Shares registered in the name of a broker, dealer, commercial bank, trust company or other nominee you must contact that institution in order to tender your Shares to us pursuant to the Offer.

Questions and requests for assistance may be directed to MacKenzie Partners, Inc. (the “Information Agent”) at its address and telephone number set forth on the back cover of this Offer to Purchase. Requests for additional copies of this Offer to Purchase, the Letter of Transmittal and other tender offer materials may be directed to the Information Agent. You may also contact your broker, dealer, commercial bank, trust company or other nominee for assistance.

This Offer to Purchase and the Letter of Transmittal contain important information, and you should read both carefully and in their entirety before making any decision with respect to the Offer.

Concentra Biosciences, LLC, a Delaware limited liability company (“Parent” or “Purchaser”), is offering to purchase (the “Offer”) all outstanding shares of voting and non-voting common stock, par value $0.01 per share (“Shares”), of IGM Biosciences, Inc., a Delaware corporation (“IGM”), for: (i) $1.247 per Share in cash (the “Cash Amount”); and (ii) one non-transferable contractual contingent value right for each Share (each, a “CVR,” and each CVR together with the Cash Amount, the “Offer Price”), all upon the terms and subject to the conditions described in this Offer to Purchase (together with any amendments or supplements hereto, this “Offer to Purchase”) and in the related Letter of Transmittal (together with any amendments or supplements thereto, the “Letter of Transmittal”). Subject to the terms of the Merger Agreement (as defined below) and the CVR Agreement (as defined below), the Offer Price will be paid net of any applicable tax withholding and without interest.

The Offer is being made pursuant to the Agreement and Plan of Merger, dated as of July 1, 2025 (together with any amendments or supplements thereto, the “Merger Agreement”), among IGM, Parent and Concentra Merger Sub V, Inc., a Delaware corporation (“Merger Sub”) and a wholly owned subsidiary of Parent, pursuant to which, after the completion of the Offer and the satisfaction or waiver of certain conditions, Merger Sub will be merged with and into IGM, without a meeting or any further action of IGM stockholders in accordance with Section 251(h) of the General Corporation Law of the State of Delaware (the “DGCL”), and IGM will be the surviving corporation and a wholly owned subsidiary of Parent (such corporation, the “Surviving Corporation,” and such merger, the “Merger”). Accordingly, Merger Sub is considered a co-offeror in the Offer. The time at which the Merger becomes effective is referred to as the “Effective Time” and the date upon which the Merger becomes effective is the “Closing Date.” Upon the terms and subject to the satisfaction or waiver of the conditions of the Offer and the Merger Agreement, including the Minimum Tender Condition (as defined below), Purchaser will accept for payment (the date and time of such acceptance, the “Offer Closing Time”) and thereafter pay for all Shares validly tendered and not properly withdrawn pursuant to the Offer promptly after the Expiration Date.

Concurrently with the execution of the Merger Agreement, and as a condition and inducement to IGM’s willingness to enter into the Merger Agreement, Tang Capital Partners, LP, a Delaware limited partnership (“TCP” or “Guarantor”) and sole member of Parent, delivered to IGM a duly executed limited guaranty (the “Limited Guaranty”), dated as of the date of the Merger Agreement, in favor of IGM, in respect of certain obligations of Parent and Merger Sub under the Merger Agreement and the CVR Agreement. Certain obligations under the Limited Guaranty are subject to a cap of $78,000,000, which includes certain Enforcement Costs, under the Merger Agreement and an amount equivalent to the CVR Proceeds (as defined below) under the CVR Agreement. Tang Capital Management, LLC, a Delaware limited liability company (“TCM”), is the sole manager of Parent and the general partner of TCP. Accordingly, TCP and TCM are considered co-offerors in the Offer. As co-offerors, TCP and TCM accept joint responsibility for the accuracy of the disclosures made in this Offer to Purchase.

The following are some questions you, as a stockholder of IGM, may have, and answers to those questions. This Summary Term Sheet highlights selected information from this Offer to Purchase and may not contain all of the information that is important to you; this Summary Term Sheet is qualified in its entirety by the more detailed descriptions and explanations contained in the Merger Agreement, the CVR Agreement, the Limited Guaranty, this Offer to Purchase and the related Letter of Transmittal. To better understand the Offer and for a complete description of the legal terms of the Offer, you should read the Merger Agreement, the CVR Agreement, the Limited Guaranty, this Offer to Purchase and the related Letter of Transmittal carefully and in their entirety. Questions or requests for assistance may be directed to MacKenzie Partners, Inc. (the “Information Agent”) at its address and telephone number, as set forth on the back cover of this Offer to Purchase. Unless otherwise indicated in this Offer to Purchase or the context otherwise requires, all references in this Offer to Purchase to “we,” “our,” or “us” refer to Parent, Merger Sub, Guarantor or TCP as the context requires.

Purchaser reserves the right to waive certain of the conditions to the Offer in their sole discretion (including the Minimum Cash Condition); provided that they may not waive the Minimum Tender Condition or the Termination Condition. The Minimum Cash Condition is subject to Closing Net Cash equal to or greater than $82,000,000.

A more detailed discussion of the conditions to consummation of the Offer is contained in the Introduction, “The Tender Offer—Section 1. Terms of the Offer” and “The Tender Offer—Section 9. Conditions of the Offer.”

Purchaser is making the Offer to acquire all outstanding Shares of IGM for: (i) $1.247 per Share in cash (the “Cash Amount”); and (ii) one non-transferable contractual contingent value right for each Share (each, a “CVR”, and each CVR together with the Cash Amount, the “Offer Price”) which represents the right to receive one or more contingent cash payments equal to a pro rata share of: (i) 80% of the net proceeds from a disposition of: (a) IGM’s product candidates known as: (A) imvotamab, an IgM-based CD20 X CD3 bispecific antibody T cell engager; (B) IGM-2644, an IgM-based CD38 X CD3 bispecific T cell engager; (C) aplitabart (or IGM-8444), a Death Receptor 5 agonist; (D) IGM-7354, a targeted IL-15/IL-15R IgM antibody; (E) IGM-8447, an IgM-based Death Receptor 5 agonist; and (F) IGM-2537, an IgM-based CD123 X CD3 bispecific antibody T cell engager; or (b) patents and patent applications owned or controlled by the Company immediately prior to the closing of the Merger and set forth on Schedule 1 of the CVR Agreement, together with all patents issuing thereon or claiming priority thereto, in each case, that occurs within one (1) year following the Merger Closing Date, and such proceeds are received by or otherwise due to Parent or any of its Affiliates within one (1) year following the Merger Closing Date; and (ii) 100% of the Closing Net Cash in excess of $82,000,000, as determined five (5) business days prior to the Merger Closing Date, each pursuant to the CVR Agreement, in each case, all upon the terms and subject to the conditions described in this Offer to Purchase and in the related Letter of Transmittal. Subject to the terms of the Merger Agreement and the CVR Agreement, the Offer Price will be paid net of any applicable tax withholding and without interest.

The Offer is being made pursuant to the Merger Agreement among IGM, Parent and Merger Sub, pursuant to which, after the completion of the Offer and the satisfaction or waiver of certain conditions, Merger Sub will be merged with and into IGM, without a meeting or any further action of IGM stockholders in accordance with Section 251(h) of the DGCL, assuming the conditions set forth in Section 251(h) of the DGCL are met, and IGM will be the Surviving Corporation and a wholly owned subsidiary of Parent. Accordingly, Merger Sub is considered a co-offeror in the Offer. Upon the terms and subject to the satisfaction or waiver of the conditions of the Offer and the Merger Agreement, including the satisfaction of the Minimum Tender Condition, Purchaser will accept for payment and thereafter pay for all Shares validly tendered and not properly withdrawn pursuant to the Offer at the Offer Closing Time.

Concurrently with the execution of the Merger Agreement, and as a condition and inducement to IGM’s willingness to enter into the Merger Agreement and the CVR Agreement, the Guarantor has duly executed and delivered to IGM the Limited Guaranty in favor of IGM and the holders of CVRs, in respect of certain obligations of Parent and Merger Sub under the Merger Agreement and certain obligations of IGM as the Surviving Corporation under the CVR Agreement. The Guarantor’s obligations under the Limited Guaranty are subject to a cap of $78,000,000, which includes certain Enforcement Costs, under the Merger Agreement and an amount equivalent to the CVR Proceeds (as defined below) under the CVR Agreement. The Guarantor and TCM are considered co-offerors in the Offer. As co-offerors, TCP and TCM accept joint responsibility for the accuracy of the disclosures made in this Offer to Purchase.

As noted in the Summary Term Sheet, there is a risk that you may receive no payments under the CVRs. Therefore, in making a decision to tender your Shares, you should understand that if the CVR does not generate any payments, the only consideration that you would receive is the Cash Amount of $1.247 per Share that is being offered pursuant to the Offer. You should base your tender decision on the Cash Amount as it may be the only consideration you receive in the Offer. On July 15, 2025, the last full trading day prior to the date of this Offer to Purchase, the closing price of IGM’s common stock as reported on Nasdaq was $1.35 per Share.

If your Shares are registered in your name and you tender directly to the Depositary and Paying Agent, you will not be obligated to pay brokerage fees or commissions or, except as otherwise provided in Instruction 6 of the Letter of Transmittal, transfer taxes on the purchase of Shares by Purchaser pursuant to the Offer. If you hold your Shares through a broker, dealer, commercial bank, trust company or other nominee you should check with such institution as to whether they charge any service fees or commissions.

We will pay all charges and expenses of the Depositary and Paying Agent and the Information Agent.

Purchaser shall not be required to accept for payment or, subject to any applicable rules and regulations of the SEC, including Rule 14e-1(c) under the Exchange Act (relating to Purchaser’s obligation to pay for or return tendered Shares promptly after the termination or withdrawal of the Offer), to pay for any Shares tendered pursuant to the Offer and, subject to the terms of the Merger Agreement, may delay the acceptance for payment of or payment for Shares or may terminate or amend the Offer, if:

Purchaser reserves the right to waive certain of the conditions to the Offer in their sole discretion (including the Minimum Cash Condition); provided that they may not waive the Minimum Tender Condition or the Termination Condition. See “The Tender Offer—Section 9. Conditions of the Offer.”

Pursuant to the terms of the Merger Agreement, the Offer and withdrawal rights will expire at one minute past 11:59 p.m. Eastern Time on August 13, 2025 (the “Expiration Date”). See “The Tender Offer —Section 1. Terms of the Offer,” “The Tender Offer—Section 9. Conditions of the Offer” and “The Tender Offer—Section 11. Certain Legal Matters; Regulatory Approvals.”

On July 1, 2025, the IGM board of directors (the “IGM Board”) at a duly called and held meeting, duly and by unanimous vote: (i) determined that the terms of the Merger Agreement, the Offer and the other transactions contemplated by the Merger Agreement and the CVR Agreement (collectively, the “Transactions”) are advisable, fair to, and in the best interests of, IGM and IGM’s stockholders; (ii) approved and declared advisable the Merger Agreement and the Transactions; (iii) authorized and approved the execution, delivery and performance by IGM of the Merger Agreement and the consummation by IGM of the Transactions; (iv) resolved to recommend that IGM’s stockholders accept the Offer and tender their shares of IGM Common Stock in the Offer; and (v) resolved that the Merger Agreement and the Merger shall be governed by and effected under Section 251(h) of the DGCL and the Merger will be consummated as soon as practicable following the Offer Closing Time.

For reasons considered by the IGM Board, see IGM’s Solicitation/Recommendation Statement on Schedule 14D-9 (the “Schedule 14D-9”) filed with the SEC on July 16, 2025 in connection with the Offer, a copy of which (without certain exhibits) is being furnished to stockholders concurrently herewith.

The Offer is being made in connection with the Merger Agreement, pursuant to which, after the completion of the Offer and the satisfaction or waiver of certain conditions, the Merger will be effected. The Merger shall become effective when a certificate of merger is filed with the Secretary of State of the State of Delaware (or at such subsequent date and time as may be agreed by Parent, IGM and Merger Sub and specified in the certificate of merger).

Pursuant to the terms of the Merger Agreement, at the Effective Time, by virtue of the Merger and without any action on the part of the holders of Shares, each outstanding Share, other than Shares held in the treasury by IGM, held by any stockholders, or owned by any beneficial owners, of IGM who are entitled to and who properly exercise appraisal rights under Delaware law or owned by Parent, Merger Sub or any other subsidiary of Parent at the commencement of the Offer and is owned by Parent, Merger Sub or any other subsidiary of Parent immediately prior to the Effective Time, will be converted into the right to receive the Offer Price, without interest and subject to any applicable tax withholding. As of immediately prior to the Effective Time, the vesting for each option to purchase Shares from IGM (“Company Stock Options,” and each a “Company Stock Option”) and, as of immediately prior to the Offer Closing Time, the vesting for each restricted stock units granted by IGM (“Company Restricted Stock Units,” and each a “Company Restricted Stock Unit”), shall be accelerated and: (i) (A) each Company Stock Option that has an exercise price per share that is less than the Cash Amount (each, an “In-the-Money Option”) that is then outstanding will be cancelled and, in exchange therefor, the holder of such cancelled In-the-Money Option will be entitled to receive in consideration of the cancellation of such In-the-Money Option (1) an amount in cash without interest, less any applicable tax withholding, equal to the product obtained by multiplying (x) the excess of the Cash Amount over the exercise price per share of Shares underlying such In-the-Money Option by (y) the number of Shares underlying such In-the-Money Option as of immediately prior to the Effective Time, and (2) one CVR for each Share underlying such In-the-Money Option, and (B) each Company Stock Option that has an exercise price per share that is equal

to or greater than the Cash Amount that is then outstanding will be cancelled for no consideration; and (ii) each Company Restricted Stock Unit that is then outstanding will be cancelled and, in exchange therefor, the holder of such cancelled Company Restricted Stock Unit will be entitled to receive in consideration of the cancellation of such Company Restricted Stock Unit (A) an amount in cash without interest, less any applicable tax withholding, equal to the Cash Amount, and (B) one CVR.

Prior to the Effective Time, IGM shall satisfy all of its notification requirements under the terms of each outstanding and unexercised Company Pre-Funded Warrant, and, at the Effective Time, each Company Pre-Funded Warrant shall be canceled and holders of such warrants will be entitled to receive: (1) an amount in cash without interest, less any applicable tax withholding, equal to the product obtained by multiplying (x) the excess of the Cash Amount over the exercise price per share of such Company Pre-Funded Warrant by (y) the number of Shares underlying such Company Pre-Funded Warrant as of immediately prior to the Effective Time; and (2) one CVR for each Share underlying such Company Pre-Funded Warrant.

The Merger Agreement is more fully described in “The Tender Offer—Section 7. Summary of the Merger Agreement and Certain Other Agreements,” which also contains a discussion of the treatment of Company Restricted Stock Units and Company Stock Options in the Merger. “Special Factors—Section 6. Material U.S. Federal Income Tax Consequences of the Offer and the Merger” below describes the material U.S. federal income tax consequences generally applicable to Holders (as defined below) whose Shares are tendered and accepted for purchase pursuant to the Offer or whose Shares are exchanged in the Merger.

Because the Merger will be consummated in accordance with Section 251(h) of the DGCL, approval of the Merger will not require a vote of IGM stockholders. Section 251(h) of the DGCL provides that stockholder approval of a merger is not required if certain requirements are met, including that: (i) the acquiring company consummates a tender offer for any and all of the outstanding stock of IGM that, absent Section 251(h) of the DGCL, would be entitled to vote on the merger; (ii) following the consummation of such tender offer, the stock irrevocably accepted for purchase pursuant to such offer and received by the depositary prior to expiration of such offer, together with the stock otherwise owned by the consummating corporation or its affiliates and any “rollover stock” (as defined in Section 251(h) of the DGCL), equals at least such percentage of the stock of IGM to be acquired that, absent Section 251(h) of the DGCL, would be required to adopt the Merger Agreement; and (iii) each outstanding share (other than “excluded stock” (as defined in Section 251(h) of the DGCL)) of the company that is subject of and not irrevocably accepted for purchase in such offer is converted in such merger into the right to receive the same amount and kind of cash, property, rights or securities paid for such shares pursuant to such offer. If the Minimum Tender Condition is satisfied and we accept Shares for payment pursuant to the Offer, we will hold a sufficient number of Shares to ensure that IGM will not be required to submit the adoption of the Merger Agreement to a vote of its stockholders. As a result of the Merger, IGM will cease to be a publicly traded company and will become a wholly owned subsidiary of Parent. See “Special Factors—Section 2. Purpose of the Offer and Plans for IGM.”

The Merger Agreement, the CVR Agreement, the Limited Guaranty, this Offer to Purchase and the related Letter of Transmittal contain important information and should be read carefully and in their entirety before any decision is made with respect to the Offer.

Background of the Offer and the Merger; Past Contacts or Negotiations between Parent, Merger Sub and IGM. The following is a description of contacts between representatives of Parent and Merger Sub with representatives of IGM that resulted in the execution of the Merger Agreement and the agreements related to the Offer. For a review of IGM’s activities relating to these contacts, please refer to IGM’s Schedule 14D-9 being mailed to stockholders with this Offer to Purchase.

On May 21, 2025, a representative of IGM senior management first contacted a representative of Parent and TCM to discuss a potential transaction with IGM.

On May 23, 2025, representatives of IGM senior management spoke with representatives of Parent and TCM to provide certain background information regarding IGM and to invite Parent to participate in IGM’s process for a potential transaction with IGM.

On May 28, 2025, IGM and Parent entered into a confidentiality agreement, which included a 1-year standstill provision and customary fall-away provisions.

Later on May 28, 2025, IGM provided certain representatives of Parent and TCM with access to IGM’s electronic data room. Subsequently, Parent and TCM conducted due diligence review of non-public information provided in IGM’s electronic data room.

On May 30, 2025, IGM senior management contacted representatives of Parent and invited Parent to submit a proposal for a potential transaction by June 10, 2025.

On June 5, 2025, representatives of Parent and TCM spoke with IGM senior management to discuss certain due diligence topics, including certain of IGM’s clinical programs and financial information of IGM. Parent and TCM continued to conduct due diligence review of non-public information provided by IGM.

On June 6, 2025, Parent submitted a non-binding proposal (the “Original Proposal”) to acquire 100% of the equity of IGM for $1.204 per share in cash plus a contingent value right (“CVR”) for each share representing the right to receive: (a) 80% of the net proceeds payable from any license or disposition of IGM’s product candidates, imvotamab, IGM-2644 and aplitabart, within 12 months of the close; and (b) 100% of the final Closing Net Cash in excess of $82.0 million, as determined within 30 days following the close. Parent’s proposal included a $6.5 million retention and was based on the availability of at least $82.0 million of closing net cash.

On June 10, 2025, IGM senior management contacted representatives of Parent to clarify IGM’s understanding of the Original Proposal.

On June 11, 2025, IGM senior management contacted representatives of Parent requesting that Parent submit a revised proposal for a potential transaction by June 13, 2025.

On June 13, 2025, Parent submitted a revised non-binding proposal (the “Revised Proposal”) to acquire 100% of the equity of IGM for $1.228 per share in cash plus a CVR for each share representing the right to receive: (a) 80% of the net proceeds payable from any license or disposition of IGM’s product candidates and/or intellectual property within 12 months of the close; and (b) 100% of the final Closing Net Cash in excess of $82.0 million, as determined within 30 days following the close. Parent’s proposal included a $5.0 million retention and was based on the availability of at least $82.0 million of closing net cash and approximately 62.7 million fully diluted shares outstanding calculated using the treasury stock method.

On June 15, 2025, IGM senior management contacted representatives of Parent to provide feedback from IGM’s Board of the Revised Proposal.

On June 17, 2025, IGM senior management contacted representatives of Parent requesting that Parent submit a revised proposal for a potential transaction by June 18, 2025.

On June 18, 2025, Parent submitted a revised non-binding proposal (the “Second Revised Proposal”) to acquire 100% of the equity of IGM for $1.246 per share in cash plus a CVR for each share representing the right to receive: (a) 80% of the net proceeds payable from any license or disposition of IGM’s product candidates and/or intellectual property within 12 months of the close; and (b) 100% of the final Closing Net Cash in excess of $82.0 million, as determined 5 business days prior to the close. Parent’s proposal included a $4.0 million retention and was based on the availability of at least $82.0 million of closing net cash and approximately 62.6 million fully diluted shares outstanding calculated using the treasury stock method.

On June 19, 2025, IGM senior management, at the direction of the IGM Board, notified Parent of IGM’s intent to enter into a definitive agreement.

On June 23, 2025, representatives of Parent and TCM spoke with IGM senior management to discuss certain financial- and clinical program-related due diligence topics.

On June 24, 2025, representatives from Parent and TCM provided IGM senior management and representatives of Wilson Sonsini Goodrich & Rosati, P.C. (“WSGR”), counsel to IGM, with initial drafts of the Merger Agreement, the CVR Agreement, the Limited Guaranty and drafts of the exhibits and schedules to be included in the Merger Agreement, including the Form of Tender and Support Agreement. The initial drafts: (a) reflected the terms of the Second Revised Proposal, except that the upfront cash amount per share was increased slightly to $1.248 per share based on an updated calculation of the fully diluted shares outstanding calculated using the treasury stock method provided to Parent by IGM; and (b) generally included customary

terms and conditions for such an agreement including, among other things, for: (i) the transaction to be structured as a cash tender offer followed immediately by a back-end merger pursuant to DGCL Section 251(h); (ii) the acceleration and cash out of certain IGM equity awards; (iii) customary exceptions to the definition of “Company Material Adverse Effect,” which generally defines the standard for certain closing risk; (iv) customary representations and warranties with respect to IGM and Parent and Merger Sub, including a representation by Parent and Merger Sub regarding the concurrent delivery of an executed limited guaranty to guarantee Parent and Merger Sub’s obligations under the Merger Agreement and the CVR Agreement; (v) IGM’s ability to provide due diligence to, and negotiate a merger agreement with, a party making an unsolicited acquisition proposal that constitutes or would reasonably be expected to lead to a superior proposal; and (vi) IGM’s ability to terminate the Merger Agreement to accept a superior proposal after providing Parent with a right to match such proposal. The Merger Agreement also included, among other things, the definition of closing net cash, which included adjustments to account for indebtedness of IGM and its subsidiaries as of closing, transaction expenses and estimated costs post-merger closing (including a cap for any legal proceedings and settlements).

On June 27, 2025, WSGR provided representatives from Parent, TCM and Gibson, Dunn & Crutcher LLP (“Gibson”), counsel to Parent, with a revised draft of the Merger Agreement, which reflected certain revisions, including: (i) a lesser termination fee payable by IGM; and (iii) raising the cap on the annual premium for the D&O insurance tail policy to be obtained.

Later on June 27, 2025, representatives from Parent and TCM provided IGM senior management and representatives of WSGR with a revised draft of the Merger Agreement, which reflected certain revisions, including: (i) a $2.425 million termination fee payable by IGM; and (iii) lowering the cap on the annual premium for the D&O insurance tail policy to be obtained.

On June 28, 2025, WSGR provided representatives from Parent, TCM and Gibson, Dunn & Crutcher LLP (“Gibson”), counsel to Parent, with a revised draft of the Merger Agreement and CVR Agreement, which reflected certain revisions, including: (i) a slightly lower upfront cash amount per share of $1.247 based on an updated calculation of the fully diluted shares outstanding calculated using the treasury stock method provided to Parent by IGM; and (ii) clarifying the definition of the CVR Products to include IGM’s product candidates known as IGM-8447 and IGM-2537 in addition to IGM’s product candidates known as imvotamab, IGM-2644, aplitabart (or IGM-8444) and IGM-7354.

From June 29, 2025 to June 30, 2025, representatives of Parent, Merger Sub and Gibson, on the one hand, and representatives of the IGM senior management team and WSGR, on the other hand, exchanged drafts of the Merger Agreement and CVR Agreement to reflect certain minor revisions, including the removal of the Form of Tender and Support Agreement.

On July 1, 2025, the Merger Agreement was signed by Parent, Merger Sub and IGM, and the Limited Guaranty was signed by TCP, Parent, Merger Sub and IGM.

On July 16, 2025, Parent commenced the Offer pursuant to the Merger Agreement, and later on July 16, 2025, IGM filed a Schedule 14D-9.

Purpose of the Offer. The purpose of the Offer and the Merger is for Parent to acquire control of, and the entire equity interest in, IGM. Pursuant to the Merger, Parent will acquire all of the stock of IGM not purchased pursuant to the Offer or otherwise.

Stockholders of IGM who sell their Shares in the Offer will cease to have any equity interest in IGM or any right to participate in its earnings and future growth.

Merger Without a Stockholder Vote. If the Offer is consummated, we do not anticipate seeking the approval of IGM’s remaining public stockholders before effecting the Merger. Section 251(h) of the DGCL provides that following consummation of a successful tender offer for a public corporation, and subject to certain statutory provisions, if the acquiring corporation owns at least the amount of shares of each class of stock of the target corporation that would otherwise be required to adopt a merger agreement for the target corporation, and the other stockholders receive the same consideration for their stock in the merger as was payable in the tender offer, the acquiring corporation can effect a merger without the action of the other stockholders of the target

corporation. Accordingly, if we consummate the Offer, we intend to effect the closing of the Merger without a vote of the stockholders of IGM in accordance with Section 251(h) of the DGCL, upon the terms and subject to the satisfaction or waiver of the conditions to the Merger, as soon as practicable after the consummation of the Offer. Accordingly, we do not expect there to be a significant period of time between the consummation of the Offer and the consummation of the Merger.

Plans for IGM. At the Effective Time, the certificate of incorporation and bylaws of IGM will each be amended and restated in their entirety pursuant to the terms of the Merger Agreement. Merger Sub’s directors immediately prior to the Effective Time will be the initial directors of the Surviving Corporation, and the officers of the Surviving Corporation shall be the respective individuals who served as the officers of Merger Sub as of immediately prior to the Effective Time, in each case, until their respective successors are duly elected and qualified, or their earlier death, resignation or removal. Each director and officer of IGM immediately prior to the Effective Time shall execute and deliver a letter effectuating his or her resignation as a member of the IGM Board or as an officer of IGM. The Surviving Corporation will not have any employees, but may engage former employees of IGM as consultants. No such terms have been agreed upon. See “The Tender Offer—Section 7. Summary of the Merger Agreement and Certain Other Agreements—Governance of the Surviving Corporation.”

At the Effective Time, Merger Sub will be merged with and into IGM, the separate existence of Merger Sub will cease, and IGM will continue as the Surviving Corporation in the Merger. IGM’s Shares will be delisted and will no longer be quoted on Nasdaq, and IGM’s obligation to file periodic reports under the Exchange Act will be suspended, and IGM will be privately held.

Except as disclosed in this Offer to Purchase, Parent and Merger Sub do not have any present plan or proposal that would result in the acquisition by any person of additional securities of IGM, the disposition of securities of IGM, an extraordinary corporate transaction, such as a merger, reorganization or liquidation, involving IGM or any of its subsidiaries or the purchase, sale or transfer of a material amount of assets of IGM or any of its subsidiaries.

Based on the determination of the IGM Board, IGM retained Houlihan Lokey Capital, Inc. (“Houlihan Lokey”) as its financial advisor in connection with the Offer, the Merger and the Transactions. In connection with this engagement, the IGM Board requested that Houlihan Lokey provide an opinion to the IGM Board as to the fairness, from a financial point of view, to the holders of Shares (other than Parent, Merger Sub and their respective affiliates) of the Offer Price proposed to be paid to such holders pursuant to the terms of the Merger Agreement. On July 1, 2025, Houlihan Lokey orally rendered to the IGM Board its opinion, which was subsequently confirmed by the delivery of a written opinion dated July 1, 2025. See “Special Factors—The Solicitation or Recommendation–Opinion of Houlihan Lokey Capital, Inc.” and Annex I of the Schedule 14D-9, which information is hereby incorporated by reference.

None of Parent, Merger Sub or the other TCP representatives engaged a financial advisor in connection with the Offer or the Merger.

According to IGM’s Annual Report on Form 10-K for the year ended December 31, 2024 filed with the SEC on March 6, 2025 and as amended on April 30, 2025, the Shares are traded on Nasdaq under the symbol “IGMS.” IGM has advised Parent that, as of the close of business on June 27, 2025, there were: (i) 60,189,151 Shares issued and outstanding (34,802,168 shares of voting common stock and 25,386,983 shares of non-voting common stock); (ii) no Shares held by IGM in its treasury; (iii) 4,308,913 Shares subject to outstanding Company Stock Options with a weighted-average exercise price of approximately $16.27 per share, 96,328 of which were In-the-Money Options with a weighted-average exercise price of approximately $1.00 per share; (iv) 1,030,014 Shares subject to outstanding Company Restricted Stock Units; and (v) 1,334,332 Shares subject to Company Pre-Funded Warrants, with an exercise price of $0.01 per Share. The following table sets forth, for the quarters indicated, the high and low sales prices per Share on Nasdaq with respect to the years ended December 31, 2024 and 2023 and the current year.

On July 15, 2025, the last full trading day prior to the date of this Offer to Purchase, the reported closing sales price per Share on Nasdaq during normal trading hours was $1.35 per Share.

IGM has never paid cash dividends on its Shares. Additionally, under the terms of the Merger Agreement, IGM is not permitted to declare or pay any dividends on or make other distributions in respect of any of its capital stock. See “The Tender Offer—Section 10. Dividends and Distributions.” Stockholders are urged to obtain a current market quotation for the Shares.

Possible Effects of the Offer on the Market for the Shares. The purchase of Shares pursuant to the Offer will reduce the number of Shares that might otherwise trade publicly and could adversely affect the liquidity and market value of the remaining Shares held by the public. The purchase of Shares pursuant to the Offer can also be expected to reduce the number of holders of Shares. We cannot predict whether the reduction in the number of Shares that might otherwise trade publicly would have an adverse or beneficial effect on the market price or marketability of the Shares or whether it would cause future market prices to be greater or less than the Offer Price.

Nasdaq Listing. Depending on the number of Shares purchased pursuant to the Offer, the Shares may no longer meet the requirements for continued listing on Nasdaq. According to the published guidelines of The Nasdaq Stock Market, LLC, Nasdaq would consider disqualifying the Shares for listing on Nasdaq if, among other possible grounds: (i) the total number of holders of record and holders of beneficial interest, taken together, in the Shares falls below 400; (ii) the bid price for a Share over a 30 consecutive business day period is less than $1.00; or (iii) (A) IGM has stockholders’ equity of less than $10 million, the number of publicly held Shares falls below 750,000, the market value of publicly held Shares over a 30 consecutive business day period is less than $5 million or there are fewer than two active and registered market makers in the Shares over a ten consecutive business day period; (B) the number of publicly held Shares falls below 1,100,000, the market value of publicly held Shares over a 30 consecutive business day period is less than $15 million, there are fewer than four active and registered market makers in the Shares over a ten consecutive business day period, or the market value of IGM’s listed securities is less than $50 million over a 30 consecutive business day period; or (C) the number of publicly held shares falls below 1,100,000, the market value of publicly held Shares over a 30 consecutive business day period is less than $15 million, there are fewer than four active and registered market makers in the Shares over a ten consecutive business day period, or IGM’s total assets and total revenue is less than $50 million each for the most recently completed fiscal year (or in two of the last three fiscal years). Nasdaq has instituted temporary waivers of certain listing rules, but there can be no guarantee as to if or when these rules will be reinstated.

On June 6, 2025, IGM received a letter from the Listing Qualifications Staff (the “Staff”) of The Nasdaq Stock Market LLC indicating that IGM was no longer in compliance with the minimum Market Value of Listed Securities (“MVLS”) of $50,000,000 required for continued listing on The Nasdaq Global Select Market pursuant to Nasdaq Listing Rule 5450(b)(2)(A). The letter also indicated that IGM will be provided with a compliance period of 180 calendar days, or until December 3, 2025, in which to regain compliance pursuant to Nasdaq Listing Rule 5450(b)(2)(A). To regain compliance with such minimum price requirement, IGM’s MVLS must close at $50,000,000 or more for a minimum of 10 consecutive business days prior to December 3, 2025. IGM’s Shares continue to trade on Nasdaq under the symbol “IGMS.”

Shares held by officers or directors of IGM, or by any beneficial owner of more than 10% of the Shares, will not be considered as being publicly held for this purpose. According to IGM, as of the close of business on June 27, 2025, there were: (i) 60,189,151 Shares issued and outstanding (34,802,168 shares of voting common stock and 25,386,983 shares of non-voting common stock); (ii) no Shares held by IGM in its treasury; (iii) 4,308,913 Shares subject to outstanding Company Stock Options with a weighted-average exercise price of approximately $16.27 per share, 96,328 of which were In-the-Money Options with a weighted-average exercise price of approximately $1,00 per share; (iv) 1,030,014 Shares subject to outstanding Company Restricted Stock Units; and (v) 1,334,332 Shares subject to Company Pre-Funded Warrants, with an exercise price of $0.01 per Share. If, as a result of the purchase of Shares pursuant to the Offer or otherwise, the Shares are delisted from Nasdaq, the market for Shares will be adversely affected.

If Nasdaq were to delist the Shares, it is possible that the Shares would continue to trade on other securities exchanges or in the over-the-counter market and that price or other quotations for the Shares would be reported by other sources. The extent of the public market for such Shares and the availability of such quotations would depend, however, upon such factors as the number of stockholders and the aggregate market value of such securities remaining at such time, the interest in maintaining a market in the Shares on the part of securities firms, the possible termination of registration under the Exchange Act as described below, and other factors. We cannot predict whether the reduction in the number of Shares that might otherwise trade publicly would have an adverse or beneficial effect on the market price for or marketability of the Shares or whether it would cause future market prices to be greater or less than the Offer Price.

Trading in the Shares will cease upon consummation of the Merger if trading has not ceased earlier as discussed above.

Exchange Act Registration. The Shares currently are registered under the Exchange Act. The purchase of the Shares pursuant to the Offer may result in the Shares becoming eligible for deregistration under the Exchange Act. Registration of the Shares may be suspended by IGM upon application to the SEC if the outstanding Shares are not listed on a “national securities exchange” and if there are fewer than 300 holders of record of Shares.

We intend to seek to cause IGM to apply for suspension of registration of the Shares as soon as possible after consummation of the Offer if the requirements for suspension of registration are met. Suspension of registration of the Shares under the Exchange Act would reduce the information required to be furnished by IGM to its stockholders and to the SEC and would make certain provisions of the Exchange Act (such as the short-swing profit recovery provisions of Section 16(b), the requirement of furnishing a proxy statement or information statement in connection with stockholders’ meetings or actions in lieu of a stockholders’ meeting pursuant to Sections 14(a) and 14(c) under the Exchange Act and the related requirement of furnishing an Annual Report to stockholders) no longer applicable with respect to the Shares. In addition, if the Shares are no longer registered under the Exchange Act, the requirements of Rule 13e-3 under the Exchange Act with respect to “going private” transactions would no longer be applicable to IGM. Furthermore, the ability of “affiliates” of IGM and persons holding “restricted securities” of IGM to dispose of such securities pursuant to Rule 144 under the Securities Act of 1933, as amended, may be impaired or eliminated. If registration of the Shares under the Exchange Act were suspended, the Shares would no longer be eligible for continued inclusion on the Board of Governors of the Federal Reserve System’s (the “Federal Reserve Board”) list of “margin securities” or eligible for stock exchange listing.

If registration of the Shares is not suspended prior to the Merger, then the registration of the Shares under the Exchange Act will be suspended following completion of the Merger.

Margin Regulations. The Shares are currently “margin securities” under the regulations of the Federal Reserve Board, which has the effect, among other things, of allowing brokers to extend credit using such Shares as collateral. Depending upon factors similar to those described above regarding listing and market quotations, following the Offer, the Shares may no longer constitute “margin securities” for the purposes of the margin regulations of the Federal Reserve Board, in which event the Shares would be ineligible as collateral for margin loans made by brokers.

The following summary describes the material U.S. federal income tax consequences generally applicable to Holders (as defined below) whose Shares are exchanged for cash and CVRs in the Offer or Merger. This

summary is for general information purposes only and is not tax advice. This summary is based on the Internal Revenue Code of 1986, as amended (the “Code”), U.S. Treasury regulations promulgated under the Code (“Treasury Regulations”), published rulings, administrative pronouncements and judicial decisions, all as in effect as of the date hereof and all of which are subject to change or differing interpretations, possibly with retroactive effect. Any such change could affect the continuing validity of this summary. This summary addresses only Holders who hold their Shares as capital assets within the meaning of Section 1221 of the Code (generally, property held for investment) and does not address all of the tax consequences that may be relevant to Holders in light of their particular circumstances or to certain types of Holders subject to special treatment under the Code, including pass-through entities (including partnerships and S corporations for U.S. federal income tax purposes) and partners or investors who hold their Shares through such entities, certain financial institutions, brokers, dealers or traders in securities or other persons that generally mark their securities to market for U.S. federal income tax purposes, insurance companies, expatriates, mutual funds, real estate investment trusts, regulated investment companies, cooperatives, tax-exempt organizations (including private foundations), retirement plans, controlled foreign corporations, passive foreign investment companies or and investors therein, persons who are subject to the alternative minimum tax, persons who hold their Shares as part of a straddle, hedge, conversion, constructive sale, synthetic security, integrated investment, or other risk-reduction transaction for U.S. federal income tax purposes, persons that have a functional currency other than the U.S. dollar, persons that own or have owned within the past five years (or are deemed to own or to have owned within the past five years) 5% or more of the outstanding Shares, Holders that exercise appraisal rights, Holders whose Shares are “qualified small business stock” within the meaning of Section 1202 of the Code or Shares to which the rollover provisions of Section 1045 of the Code apply, and persons who acquired their Shares upon the vesting and cancellation of Company Stock Options or Company Restricted Stock Units in connection with the Merger or otherwise as compensation. This summary does not address any U.S. federal estate, gift, or other non-income tax consequences, the effects of the Medicare contribution tax on net investment income, or any state, local, or non-U.S. tax consequences.

As used in this summary, the term “U.S. Holder” means a beneficial owner of Shares that, for U.S. federal income tax purposes, is: (i) an individual who is a citizen or resident of the United States; (ii) a corporation, or other entity classified as a corporation for U.S. federal income tax purposes, that is created or organized in or under the laws of the United States or any state thereof or the District of Columbia; (iii) an estate, the income of which is subject to U.S. federal income taxation regardless of its source; or (iv) a trust if (A) a court within the United States is able to exercise primary supervision over the administration of the trust and one or more “U.S. persons” (within the meaning of Section 7701(a)(30) of the Code) has the authority to control all substantial decisions of the trust or (B) the trust has a valid election in effect under applicable Treasury Regulations to be treated as a U.S. person.

As used in this summary, the term “Non-U.S. Holder” means a beneficial owner of Shares that is neither a U.S. Holder nor a partnership for U.S. federal income tax purposes, and the term “Holder” or “Holders” means a U.S. Holder or a Non-U.S. Holder.

If a partnership (including any entity or arrangement classified as a partnership for U.S. federal income tax purposes) exchanges Shares for cash and CVRs pursuant to the Offer or the Merger, the tax treatment of a partner in the partnership generally will depend upon the status of the partner, the activities of the partnership, and certain determinations made at the partner level. The partnership and partners of the partnership holding Shares should consult their tax advisors regarding the particular tax consequences of exchanging Shares for cash and CVRs pursuant to the Offer or the Merger applicable to them.

We have not sought, and do not expect to seek, a ruling from the IRS as to any U.S. federal income tax consequence described herein, and no assurance can be given that the IRS will not take a position contrary to the discussion below, or that a court will not sustain any challenge by the IRS in the event of litigation.

Holders are urged to consult their tax advisors to determine the tax consequences to them of exchanging Shares for cash and CVRs pursuant to the Offer or the Merger in light of their particular circumstances.

Tax Considerations for U.S. Holders. The exchange of Shares for cash and CVRs pursuant to the Offer or the Merger will be a taxable transaction for U.S. federal income tax purposes. The amount of gain or loss a U.S. Holder recognizes, and the timing and character of a portion of such gain or loss, depends on the

U.S. federal income tax treatment of the CVRs, with respect to which there is a significant amount of uncertainty. The installment method of reporting any gain attributable to the receipt of a CVR generally will not be available with respect to the disposition of Shares pursuant to the Offer or the Merger because the Shares are traded on an established securities market.

There is no legal authority directly addressing the U.S. federal income tax treatment of the receipt of the CVRs or payments received thereunder in connection with the Offer or the Merger. The receipt of the CVRs as part of the Offer or the Merger consideration might be treated as a “closed transaction” or as an “open transaction” for U.S. federal income tax purposes, or in some other manner, and such questions are inherently factual in nature. Accordingly, Holders are urged to consult with their tax advisors regarding this issue.

Pursuant to Treasury Regulations addressing contingent payment obligations analogous to the CVRs, if the fair market value of the CVR is “reasonably ascertainable,” a U.S. Holder should treat the transaction as a “closed transaction” and treat the fair market value of the CVRs as part of the consideration received in the Offer or the Merger for purposes of determining gain or loss. On the other hand, if the fair market value of the CVRs cannot be reasonably ascertained, a U.S. Holder may treat the transaction as an open transaction for purposes of determining gain or loss. These Treasury Regulations state that only in “rare and extraordinary” cases would the value of contingent payment obligations not be reasonably ascertainable. As noted above, there is no authority directly addressing whether contingent payment rights with characteristics similar to the rights under a CVR should be treated as “open transactions” or “closed transactions,” and such question is inherently factual in nature. The CVRs also may be treated as contract rights, debt instruments or deferred payment contract rights for U.S. federal income tax purposes, which would affect the amount, timing, and character of any gain, income or loss with respect to the CVRs. We urge you to consult your own tax advisor with respect to the proper characterization of the receipt of, and payments made with respect to, a CVR.

As a result, we cannot express a definitive conclusion as to the U.S. federal income tax treatment of receipt of the CVRs or receipt of any payment pursuant to the CVRs. However, we intend to treat the receipt of the CVRs as a closed transaction and payments received pursuant to the CVRs as amounts realized on the disposition (or partial disposition) of the CVRs. We cannot give any assurance that the IRS would not assert, or that a court would not sustain, a position contrary to this treatment. In such event, the tax consequences of the receipt of CVRs and/or payments with respect to the CVRs could differ materially from those summarized below (including, potentially, a portion or all of payments made with respect to the CVRs giving rise to ordinary income, rather than capital gain).

No opinion of counsel or ruling has been or will be sought from the IRS regarding the tax treatment of the CVRs and payments received pursuant to the CVRs. We cannot give any assurance that the IRS would not assert, or that a court would not sustain, a position contrary to this treatment. In such event, the tax consequences of the receipt of CVRs and/or payments with respect to the CVRs could differ materially from those summarized below (including, potentially, a portion or all of payments made with respect to the CVRs giving rise to ordinary income, rather than capital gain). No opinion of counsel or ruling has been or will be sought from the IRS regarding the tax treatment of the CVRs.

Treatment as a Closed Transaction. If the receipt of a CVR is part of a closed transaction, a U.S. Holder generally would recognize capital gain or loss on a sale of Shares for the Offer Price pursuant to the Offer or an exchange of Shares for the Offer Price pursuant to the Merger, in an amount equal to the difference, if any, between: (i) the sum of the Cash Amount received plus the fair market value (determined as of the closing of the Offer or the Effective Time, as the case may be) of any CVRs received; and (ii) the U.S. Holder’s adjusted tax basis in the Shares sold or exchanged. Gain or loss generally would be calculated separately for each block of Shares (that is, Shares acquired at the same cost in a single transaction) tendered pursuant to the Offer or exchanged for the Offer Price pursuant to the Merger. The proper method to determine the fair market value of a CVR is not clear, but it is possible that the trading value of IGM’s common stock would be considered along with other factors in making that determination. Any capital gain or loss recognized will be long-term capital gain or loss if the U.S. Holder’s holding period for such Shares exceeds one year. The deductibility of capital losses is subject to limitations.

A U.S. Holder’s initial tax basis in a CVR received in either the Offer or the Merger would equal the fair market value of such CVR (determined as of the closing of the Offer or the Effective Time, as the case may be) as determined for U.S. federal income tax purposes. The holding period for a CVR would begin on the day

following the date of the closing of the Offer or the Effective Time, as the case may be. We intend to cooperate with the Paying Agent to send to each U.S. Holder an IRS Form 1099-B reflecting our determination of the fair market value of the CVRs issued in the Offer or Merger. Such determination is not binding on the IRS as to the stockholder’s tax treatment or the fair market value of the CVRs, and each Holder should be aware that the IRS may challenge the fair market value of the CVRs reported by such Holder.

As noted above, there is no authority directly addressing the U.S. federal income tax treatment of contingent payment rights with characteristics similar to the rights under the CVRs, and, therefore, the amount, timing and character of any gain, income or loss with respect to the CVRs is uncertain. For example, payments with respect to the CVRs could be treated as payments with respect to a sale or exchange of a capital asset or as giving rise to ordinary income. It is also possible that, were a payment to be treated as being with respect to the sale of a capital asset, a portion of such payment would constitute imputed interest, as described below. We intend to treat any payment received by a U.S. Holder in respect of such CVRs (except to the extent any portion of such payment is required to be treated as imputed interest, as described below) as an amount realized on the disposition of the CVR by the U.S. Holder. Assuming that this method of reporting is correct, a U.S. Holder should recognize gain or loss equal to the difference between such payment (less any portion of such payment required to be treated as imputed interest, as described below) and the U.S. Holder’s adjusted tax basis in the CVR. The gain or loss should be long-term capital gain or loss if the U.S. Holder has held the CVR for more than one year at the time of such payment. Additionally, a U.S. Holder may recognize capital loss to the extent of any remaining basis after the expiration of any right to cash payments under such U.S. Holder’s CVR. The deductibility of capital losses is subject to limitations.

Treatment as an Open Transaction. If the receipt of a CVR pursuant to the Offer or the Merger is treated under the open transaction method of accounting for U.S. federal income tax purposes, the fair market value of the CVR will not be treated as additional consideration for the Shares at the time the CVR is received, and the U.S. Holder will not have any tax basis in the CVR. Instead, the U.S. Holder will take payments pursuant to the CVRs into account when made or deemed made in accordance with the U.S. Holder’s regular method of accounting for U.S. federal income tax purposes. Generally, a portion of such payments will be treated as imputed interest, as described in more detail below, and the balance as additional consideration recognized in exchange for the Shares.

If the receipt of a CVR is part of an open transaction then, although not entirely clear, the sum of the Cash Amount received for a U.S. Holder’s Shares and the portion of the payments pursuant to the CVR that is not treated as imputed interest will generally be applied first against a U.S. Holder’s adjusted tax basis in the Shares and any excess treated as gain. A U.S. Holder will recognize capital loss with respect to a Share to the extent that the holder’s adjusted tax basis in such Share exceeds the sum of the Cash Amount received for a U.S. Holder’s Shares and the cash received pursuant to the CVR that is not treated as imputed interest, although it is possible that such U.S. Holder may not be able to recognize such loss until the resolution of all contingencies under the CVRs or possibly until such U.S. Holder’s abandonment of the U.S. Holder’s CVR. Any such capital gain or loss will be long-term capital gain or loss if the U.S. Holder’s holding period in the Share exceeds one year. The deductibility of capital losses is subject to limitations. Gain or loss generally will be determined separately for each block of Shares (that is, Shares of the same class acquired at the same cost on the same day) exchanged pursuant to the Offer or the Merger.

Imputed Interest. A portion of any payments pursuant to the CVRs that are made more than six months after the closing of the Offer or the Effective Time, as the case may be, may be treated as imputed interest, which would be ordinary income to the U.S. Holder of a CVR. The portion of any payment made with respect to a CVR treated as imputed interest under Section 483 of the Code will be determined at the time such payment is made and generally should equal the excess of: (i) the amount of the payment in respect of the CVRs; over (ii) the present value of such amount as of the closing of the Offer or the Effective Time, as the case may be, calculated using the applicable federal rate as the discount rate. The applicable federal rate is published monthly by the IRS. A U.S. Holder must include in its taxable income interest imputed pursuant to Section 483 of the Code (if any) using such Holder’s regular method of accounting for U.S. federal income tax purposes.

U.S. Holders are urged to consult their tax advisors with respect to the proper characterization of the CVRs and the tax considerations thereof (including any future payments made under the CVRs).

Tax Considerations for Non-U.S. Holders. Any gain realized by a Non-U.S. Holder upon the tender of Shares pursuant to the Offer or the exchange of Shares pursuant to the Merger, as the case may be, generally will not be subject to U.S. federal income tax unless:

Generally, if payments are made to a Non-U.S. Holder with respect to a CVR, such Non-U.S. Holder may be subject to withholding at a rate of 30% (or a lower applicable treaty rate) on the portion of any such payments treated as imputed interest (as discussed above under “U.S. Holders—Imputed Interest”), unless such Non-U.S. Holder establishes its entitlement to exemption from or a reduced rate of withholding under an applicable tax treaty by providing the appropriate documentation (generally, IRS Form W-8BEN or W-8BEN-E or other applicable IRS Form W-8) to the applicable withholding agents. As discussed above, the tax treatment of the CVRs is unclear, and it is possible that the Depositary and Paying Agent or other withholding agent may withhold additional amounts on payments with respect to the CVRs.

Amounts treated as imputed interest that are effectively connected with a Non-U.S. Holder’s conduct of a trade or business in the United States and, if required by an applicable income tax treaty, are attributable to a permanent establishment in the United States, are generally taxed in the manner applicable to U.S. Holders, as described above. In such cases, the Non-U.S. Holder will not be subject to withholding so long as such Non-U.S. Holder complies with applicable certification and disclosure requirements. In addition, interest received by a non-U.S. corporation that are effectively connected with the conduct of a trade or business in the United States may be subject to a branch profits tax at a 30% rate, or a lower rate specified in an applicable income tax treaty. See “The Tender Offer—Section 3. Procedures for Tendering Shares” for information regarding the application of U.S. federal income tax withholding to payments made to Non-U.S. Holders.

If any amounts withheld exceed the Non-U.S. Holder’s U.S. federal income tax liability, such Non-U.S. Holder may obtain a refund or credit of any excess amounts withheld by filing an appropriate claim for a refund with the IRS.

Non-U.S. Holders are urged to consult their own tax advisors regarding the particular tax consequences to them of exchanging Shares in the Offer or the Merger, including the application of the 30% U.S. federal withholding tax, their potential eligibility for a reduced rate of, or exemption from, such withholding tax, and their potential eligibility for, and procedures for claiming, a refund of any such withholding tax.

Information Reporting, Backup Withholding and FATCA. Information reporting generally will apply to payments to a Holder pursuant to the Offer or the Merger (including payments with respect to a CVR), unless such Holder is an entity that is exempt from information reporting and, when required, properly demonstrates its

eligibility for exemption. Any payment to a U.S. Holder that is subject to information reporting generally will also be subject to backup withholding, unless such U.S. Holder: (i) provides the appropriate documentation (generally, IRS Form W-9) to the applicable withholding agent certifying that, among other things, its taxpayer identification number is correct, or otherwise establishes an exemption; and (ii) with respect to payments on the CVRs, provides the rights agent with the certification documentation in clause (i) of this sentence or otherwise establishes an exemption from backup withholding tax.

The information reporting and backup withholding rules that apply to payments to a Holder pursuant to the Offer and Merger generally will not apply to payments to a Non-U.S. Holder if such Non-U.S. Holder certifies under penalties of perjury that it is not a U.S. person (generally by providing an IRS Form W-8BEN, IRS Form W-8BEN-E or other applicable IRS Form W-8) or otherwise establishes an exemption. Non-U.S. Holders should consult their own tax advisors to determine which IRS Form W-8 is appropriate.

As discussed above, we intend to send to each U.S. Holder an IRS Form 1099-B treating the Offer or the Merger, as applicable, as a “closed transaction” for U.S. federal income tax purposes. Accordingly, U.S. Holders that treat the Offer or the Merger, as applicable, as an “open transaction” for U.S. federal income tax purposes are urged to consult their own tax advisors regarding how to accurately report their income under this method. Certain Holders (including corporations) generally are not subject to backup withholding. Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules generally will be allowed as a refund or a credit against a U.S. Holder’s U.S. federal income tax liability if the required information is properly and timely furnished by such U.S. Holder to the IRS.

Under the “Foreign Account Tax Compliance Act” provisions of the Code, related U.S. Treasury guidance and related intergovernmental agreements (“FATCA”), the Depositary and Paying Agent or another applicable withholding agent will be required to withhold tax at a rate of 30% on payments of amounts treated as interest pursuant to U.S. tax law to any Non-U.S. Holder that fails to meet prescribed certification requirements. In general, no such withholding will be required with respect to a person that timely provides certifications that establish an exemption from FATCA withholding on a valid IRS Form W-8. A Non-U.S. Holder may be able to claim a credit or refund of the amount withheld under certain circumstances. Under currently proposed Treasury Regulations, FATCA withholding would no longer apply to payments that are treated as gross proceeds from the sale or other disposition of property of a type that can generate U.S. source interest or dividends, including the Shares. Taxpayers generally may rely on these proposed Treasury Regulations until final Treasury Regulations are issued. Each Non-U.S. Holder should consult its own tax advisor regarding the application of FATCA to the CVRs.

THE FOREGOING SUMMARY DOES NOT PURPORT TO BE A COMPLETE DISCUSSION OF THE POTENTIAL TAX CONSEQUENCES OF THE OFFER OR THE MERGER OR THE OWNERSHIP OF CVRS. EACH HOLDER SHOULD CONSULT ITS OWN TAX ADVISOR REGARDING THE U.S. FEDERAL, STATE AND LOCAL AND NON-U.S. INCOME, ESTATE AND OTHER TAX CONSIDERATIONS RELATING TO THE OFFER AND MERGER IN LIGHT OF ITS PARTICULAR CIRCUMSTANCES. THE U.S. FEDERAL, STATE, LOCAL AND NON-U.S. INCOME AND OTHER TAX CONSEQUENCES TO HOLDERS OR BENEFICIAL OWNERS OF COMPANY STOCK OPTIONS OR COMPANY RESTRICTED STOCK UNITS PARTICIPATING IN THE MERGER WITH RESPECT TO SUCH COMPANY STOCK OPTIONS OR COMPANY RESTRICTED STOCK UNITS ARE NOT DISCUSSED HEREIN, AND SUCH HOLDERS OR BENEFICIAL OWNERS OF COMPANY STOCK OPTIONS OR COMPANY RESTRICTED STOCK UNITS ARE STRONGLY ENCOURAGED TO CONSULT THEIR OWN TAX ADVISORS REGARDING SUCH TAX CONSEQUENCES. NOTHING IN THIS SUMMARY IS INTENDED TO BE, OR SHOULD BE CONSTRUED AS, TAX ADVICE.

Upon the terms and subject to the prior satisfaction or waiver of the conditions of the Offer (including, if the Offer is extended or amended, the terms and conditions of any extension or amendment), we will accept for payment, purchase and pay for all Shares validly tendered prior to the expiration of the Offer, and not properly withdrawn in accordance with the procedures set forth in “The Tender Offer—Section 4. Withdrawal Rights.” The Offer will expire at one minute after 11:59 p.m. Eastern Time on August 13, 2025, unless extended in accordance with the terms of the Merger Agreement, in which event the term “Expiration Date” will mean the date to which the Expiration Date of the Offer is so extended.

Purchaser is offering to pay: (i) $1.247 per Share in cash (the “Cash Amount”); and (ii) one non-transferable contractual contingent value right for each Share (each, a “CVR”, and each CVR together with the Cash Amount, the “Offer Price”) which represents the right to receive one or more contingent cash payments equal to a pro rata share of: (i) 80% of the net proceeds from a disposition of (a) IGM’s product candidates known as: (A) imvotamab, an IgM-based CD20 X CD3 bispecific antibody T cell engager; (B) IGM-2644, an IgM-based CD38 X CD3 bispecific T cell engager; (C) aplitabart (or IGM-8444), a Death Receptor 5 agonist; (D) IGM-7354, a targeted IL-15/IL-15R IgM antibody; (E) IGM-8447, an IgM-based Death Receptor 5 agonist; and (F) IGM-2537, an IgM-based CD123 X CD3 bispecific antibody T cell engager; or (b) patents and patent applications owned or controlled by the Company immediately prior to the closing of the Merger and set forth on Schedule 1 of the CVR Agreement, together with all patents issuing thereon or claiming priority thereto, in each case, that occurs within one (1) year following closing the Merger Closing Date, and such proceeds are received by or otherwise due to Parent or any of its Affiliates within one (1) year following the Merger Closing Date; and (ii) 100% of the Closing Net Cash in excess of $82,000,000, as determined five (5) business days prior to the Merger Closing Date, each pursuant to the CVR Agreement, in each case, all upon the terms and subject to the conditions described in this Offer to Purchase and in the related Letter of Transmittal. Subject to the terms of the Merger Agreement and CVR Agreement, the Offer Price will be paid net of any applicable tax withholding and without interest.

As noted in the Summary Term Sheet, there is a risk that you may receive no payments under the CVRs. Therefore, in making a decision to tender your Shares, you should understand that if the CVR does not generate any payments, the only consideration that you would receive is the Cash Amount of $1.247 per Share that is being offered pursuant to the Offer. You should base your tender decision on the Cash Amount as it may be the only consideration you receive in the Offer. On July 15, 2025, the last full trading day prior to the date of this Offer to Purchase, the closing price of IGM’s common stock as reported on Nasdaq was $1.35 per Share.

The Offer is conditioned upon the satisfaction of the Minimum Tender Condition and the other conditions described in “The Tender Offer—Section 9. Conditions of the Offer.” We may terminate the Offer without purchasing any Shares if certain events described in “The Tender Offer—Section 7. Summary of the Merger Agreement and Certain Other Agreements—Summary of the Merger Agreement—Termination” occur.

Purchaser expressly reserves the right, in its sole discretion, to: (i) waive, in whole or in part, any Offer Condition (including the Minimum Cash Condition), other than the Minimum Tender Condition or the Termination Condition; and/or (ii) modify the terms of the Offer in a manner not inconsistent with the Merger Agreement, except that IGM’s consent is required for Purchaser to:

Upon the terms and subject to the satisfaction or waiver of the conditions of the Offer and the Merger Agreement, including the Minimum Tender Condition, Purchaser will accept for payment and thereafter pay for all Shares validly tendered and not properly withdrawn pursuant to the Offer promptly after the Expiration Date. The Offer will not permit Shares to be tendered pursuant to guaranteed delivery procedures.

If, on or before the Expiration Date, we increase the consideration being paid for Shares accepted for payment in the Offer, such increased consideration will be paid to all stockholders whose Shares are purchased in the Offer, whether or not such Shares were tendered before the announcement of the increase in consideration. We also expressly reserve the right to modify the terms of the Offer, subject to compliance with the Exchange Act, the Merger Agreement and the restrictions identified in paragraphs (A) through (H) above.

The Merger Agreement provides that, unless the Merger Agreement has been validly terminated in accordance with its terms, (A) Purchaser may elect to (and if so requested by IGM, Purchaser will) extend the Offer for one or more consecutive increments of such duration as requested by IGM, but not more than 10 business days each, if at the scheduled Expiration Date of the Offer (i) any of the Offer Conditions (as set forth in “The Tender Offer—Section 9. Conditions of the Offer”) shall not have been satisfied or waived, until such time as such conditions shall have been satisfied or waived or (ii) the determination of the Closing Net Cash and resolution of any dispute thereof has not been finalized in accordance with the terms of the Merger Agreement, and (B) Purchaser shall extend the Offer for the minimum period required by any rule, regulation or interpretation or position of the SEC or the staff thereof or Nasdaq applicable to the Offer; provided that Purchaser shall not, and shall not be required to, extend the Offer beyond 11:59 p.m., Eastern Time, on September 29, 2025 (the “Outside Date”).

See “The Tender Offer—Section 7. Summary of the Merger Agreement and Certain Other Agreements.”

Except as set forth above, there can be no assurance that we will be required under the Merger Agreement to extend the Offer. During any extension of the initial offering period pursuant to the paragraphs above, all Shares previously tendered and not withdrawn will remain subject to the Offer and subject to withdrawal rights. See “The Tender Offer—Section 4. Withdrawal Rights.”

Without IGM’s consent, there will not be a subsequent offering period for the Offer.

If, subject to the terms of the Merger Agreement, we make a material change in the terms of the Offer or the information concerning the Offer, or if we waive a material condition of the Offer, we will disseminate additional tender offer materials and extend the Offer if and to the extent required by Rules 14d-3(b)(1), 14d-4(d), 14d-6(c) and l4e-1 under the Exchange Act or otherwise. The minimum period during which a tender offer must remain open following material changes in the terms of the tender offer or the information concerning the tender offer, other than a change in the consideration offered or a change in the percentage of securities sought, will depend upon the facts and circumstances, including the relative materiality of the terms or information changes. With respect to a change in the consideration offered or a change in the percentage of securities sought, a tender offer generally must remain open for a minimum of 10 business days following the disclosure of such change to stockholders to allow for adequate disclosure to stockholders.

We expressly reserve the right, in our sole discretion, subject to the terms and upon the conditions of the Merger Agreement and the applicable rules and regulations of the SEC, to not accept for payment any Shares if, at the expiration of the Offer, any of the conditions to the Offer set forth in “The Tender Offer—Section 9. Conditions of the Offer” have not been satisfied. Under certain circumstances, Parent and Merger Sub may terminate the Merger Agreement and the Offer.

Any extension, waiver or amendment of the Offer or termination of the Offer will be followed, as promptly as practicable, by public announcement thereof, such announcement in the case of an extension to be issued not later than 9:00 a.m. Eastern Time on the next business day after the Expiration Date in accordance with the public announcement requirements of Rules 14d-3(b)(1), 14d-4(d) and 14e-1(d) under the Exchange Act. Without limiting our obligation under such rule or the manner in which we may choose to make any public announcement, we currently intend to make announcements by issuing a press release to the PR Newswire (or such other national media outlet or outlets we deem prudent) and making any appropriate filing with the SEC.

Promptly following the purchase of Shares in the Offer, we expect to complete the Merger without a vote of the stockholders of IGM pursuant to Section 251(h) of the DGCL.

IGM has agreed to provide us with its list of stockholders and security position listings for the purpose of disseminating the Offer to holders of Shares. This Offer to Purchase and the related Letter of Transmittal will be mailed to record holders of Shares whose names appear on IGM’s stockholder list and will be furnished to brokers, dealers, commercial banks, trust companies and similar persons whose names, or the names of whose nominees, appear on the stockholder list or, if applicable, who are listed as participants in a clearing agency’s security position listing, for subsequent transmittal to beneficial owners of Shares.

Subject to the satisfaction or waiver of all the conditions to the Offer set forth in “The Tender Offer—Section 9. Conditions of the Offer,” we will immediately after the Expiration Date irrevocably accept for payment all Shares tendered (and not validly withdrawn) pursuant to the Offer and, promptly after the Expiration Date (and in any event within three business days), pay for such Shares.

In all cases, payment for Shares tendered and accepted for payment pursuant to the Offer will be made only after timely receipt by the Depositary and Paying Agent of: (i) certificates (if any) representing such Shares or confirmation of the book-entry transfer of such Shares into the Depositary and Paying Agent’s account at DTC pursuant to the procedures set forth in “The Tender Offer—Section 3. Procedures for Tendering Shares;” (ii) a Letter of Transmittal, properly completed and duly executed, with any required signature guarantees (or, in the case of a book-entry transfer, an Agent’s Message (as defined below) in lieu of the Letter of Transmittal); and (iii) any other documents required by the Letter of Transmittal or any other customary documents required by Depositary and Paying Agent. See “The Tender Offer—Section 3. Procedures for Tendering Shares.”

For purposes of the Offer, if and when Purchaser gives written notice to the Depositary and Paying Agent of its acceptance for payment of such Shares pursuant to the Offer, then Purchaser has accepted for payment and thereby purchased Shares validly tendered and not properly withdrawn pursuant to the Offer. Upon the terms and subject to the conditions of the Offer, payment for Shares accepted for payment pursuant to the Offer will be made by deposit of the purchase price therefor with the Depositary and Paying Agent, which will act as agent for the tendering stockholders for purposes of receiving payments from us and transmitting such payments to the tendering stockholders. Under no circumstances will interest be paid on the Offer Price for Shares, regardless of any extension of the Offer or any delay in payment for Shares.

If any tendered Shares are not accepted for payment pursuant to the terms and conditions of the Offer for any reason, or if certificates are submitted for more Shares than are tendered, certificates for such unpurchased Shares will be returned (or new certificates for the Shares not tendered will be sent), without expense to the tendering stockholder (or, in the case of Shares tendered by book-entry transfer into the Depositary and Paying Agent’s account at DTC pursuant to the procedures set forth in “The Tender Offer—Section 3. Procedures for Tendering Shares,” such Shares will be credited to an account maintained with DTC) promptly following expiration or termination of the Offer.

Valid Tender of Shares. To validly tender Shares pursuant to the Offer, a properly completed and duly executed Letter of Transmittal in accordance with the instructions of the Letter of Transmittal, with any required signature guarantees, or an Agent’s Message (as defined below) in connection with a book-entry delivery of Shares, and any other documents required by the Letter of Transmittal and any other customary documents required by the Depositary and Paying Agent, must be received by the Depositary and Paying Agent at one of its addresses set forth on the back cover of this Offer to Purchase prior to the expiration of the Offer and either:

(i) certificates (if any) representing Shares tendered must be delivered to the Depositary and Paying Agent; or (ii) such Shares must be properly delivered pursuant to the procedures for book-entry transfer described below and a confirmation of such delivery received by the Depositary and Paying Agent (which confirmation must include an Agent’s Message (as defined below)), in each case, prior to the Expiration Date. The term “Agent’s Message” means a message, transmitted by DTC to, and received by, the Depositary and Paying Agent and forming a part of a Book-Entry Confirmation (as defined below), which states that DTC has received an express acknowledgment from the participant in DTC tendering the Shares which are the subject of such Book-Entry Confirmation (as defined below) that such participant has received and agrees to be bound by the terms of the Letter of Transmittal and that Purchaser may enforce such agreement against the participant.

Book-Entry Transfer. The Depositary and Paying Agent will take steps to establish and maintain an account with respect to the Shares at DTC for purposes of the Offer. Any financial institution that is a participant in DTC’s systems may make a book-entry transfer of Shares by causing DTC to transfer such Shares into the Depositary and Paying Agent’s account in accordance with DTC’s procedures for such transfer. An Agent’s Message in lieu of the Letter of Transmittal, and any other required documents, must, in any case, be transmitted to and received by the Depositary and Paying Agent at one of its addresses set forth on the back cover of this Offer to Purchase prior to the Expiration Date. The confirmation of a book-entry transfer of Shares into the Depositary and Paying Agent’s account at DTC as described above is referred to herein as a “Book-Entry Confirmation.”

Delivery of documents to DTC in accordance with DTC’s procedures does not constitute delivery to the Depositary and Paying Agent.

Signature Guarantees. Except as otherwise provided below, all signatures on a Letter of Transmittal must be guaranteed by a financial institution (including most commercial banks, savings and loan associations and brokerage houses) that is a member in good standing of a recognized Medallion Program approved by the Securities Transfer Association, Inc., including the Security Transfer Agents Medallion Program, the New York Stock Exchange Medallion Signature Program and the Stock Exchanges Medallion Program (each, an “Eligible Institution”). Signatures on a Letter of Transmittal need not be guaranteed: (i) if the Letter of Transmittal is signed by the registered owner(s) of Shares tendered therewith and such registered owner has not completed the box entitled “Special Payment Instructions” or the box entitled “Special Delivery Instructions” on the Letter of Transmittal; or (ii) if such Shares are tendered for the account of an Eligible Institution. See Instructions 1 and 5 of the Letter of Transmittal. If the certificates for Shares are registered in the name of a person other than the signer of the Letter of Transmittal, or if payment is to be made or certificates for Shares not tendered or not accepted for payment are to be returned to a person other than the registered owner of the certificates surrendered, then signature(s) must be guaranteed as described above. See Instructions 1 and 5 of the Letter of Transmittal.

If certificates representing Shares are forwarded separately to the Depositary and Paying Agent, a properly completed and duly executed Letter of Transmittal must accompany each delivery of certificates.

THE METHOD OF DELIVERY OF SHARES, THE LETTER OF TRANSMITTAL AND ALL OTHER REQUIRED DOCUMENTS, INCLUDING DELIVERY THROUGH DTC, IS AT THE ELECTION AND RISK OF THE TENDERING STOCKHOLDER. DELIVERY OF ALL SUCH DOCUMENTS WILL BE DEEMED MADE ONLY WHEN ACTUALLY RECEIVED BY THE DEPOSITARY (INCLUDING, IN THE CASE OF A BOOK-ENTRY TRANSFER, BY BOOK-ENTRY CONFIRMATION). IF SUCH DELIVERY IS BY MAIL, IT IS RECOMMENDED THAT ALL SUCH DOCUMENTS BE SENT BY PROPERLY INSURED REGISTERED MAIL WITH RETURN RECEIPT REQUESTED. IN ALL CASES, SUFFICIENT TIME SHOULD BE ALLOWED TO ENSURE TIMELY DELIVERY.

Other Requirements. Purchaser will pay for Shares tendered (and not validly withdrawn) pursuant to the Offer only after timely receipt by the Depositary and Paying Agent of: (i) certificates (if any) for (or a timely Book-Entry Confirmation with respect to) such Shares; (ii) a Letter of Transmittal, properly completed and duly executed, with any required signature guarantees (or, in the case of a book-entry transfer, an Agent’s Message in lieu of the Letter of Transmittal); and (iii) any other documents required by the Letter of Transmittal or any other customary documents required by the Depositary and Paying Agent. Under no circumstances will Purchaser

pay interest on the purchase price of Shares, regardless of any extension of the Offer or any delay in making such payment. If your Shares are held in street name (i.e., through a broker, dealer, commercial bank, trust company or other nominee), your Shares can be tendered by your nominee by book-entry transfer through the Depositary and Paying Agent.

Binding Agreement. Our acceptance for payment of Shares tendered pursuant to one of the procedures described above will constitute a binding agreement between the tendering stockholder and us upon the terms and subject to the conditions of the Offer.

Appointment as Proxy. By executing and delivering a Letter of Transmittal as set forth above (or, in the case of a book-entry transfer, by delivery of an Agent’s Message in lieu of a Letter of Transmittal), the tendering stockholder irrevocably appoints Purchaser’s designees as such stockholder’s proxies, each with full power of substitution, to the full extent of such stockholder’s rights with respect to the Shares tendered by such stockholder and accepted for payment by us and with respect to any and all other Shares or other securities issued or issuable in respect of such Shares on or after the date of the Merger Agreement. All such proxies and powers of attorney will be considered coupled with an interest in the tendered Shares. Such appointment is effective when, and only to the extent that, we accept for payment Shares tendered by such stockholder as provided herein. Upon the effectiveness of such appointment, all prior powers of attorney, proxies and consents given by such stockholder will be revoked, and no subsequent powers of attorney, proxies and consents may be given (and, if given, will not be deemed effective). Our designees will, with respect to the Shares or other securities and rights for which the appointment is effective, be empowered to exercise all voting and other rights of such stockholder as they, in their sole discretion, may deem proper at any annual, special, adjourned or postponed meeting of the stockholders of IGM, by written consent in lieu of any such meeting or otherwise. We reserve the right to require that, in order for Shares to be deemed validly tendered, immediately upon our payment for such Shares we must be able to exercise full voting, consent and other rights to the extent permitted under applicable law with respect to such Shares and other securities, including voting at any meeting of stockholders or executing a written consent concerning any matter.

Determination of Validity. All questions as to the validity, form, eligibility (including time of receipt) and acceptance of any tender of Shares will be determined by us in our sole and absolute discretion (which may delegate power in whole or in part to the Paying and Depositary Agent), which determination will be final and binding, subject to the rights of the tendering holders of Shares to challenge our determination in a court of competent jurisdiction. Purchaser reserves the absolute right to reject any and all tenders determined by us not to be in proper form or the acceptance for payment of or payment for which may, in our opinion, be unlawful. Purchaser also reserves the absolute right to waive any defect or irregularity in the tender of any Shares of any particular stockholder whether or not similar defects or irregularities are waived in the case of any other stockholder. No tender of Shares will be deemed to have been validly made until all defects and irregularities relating thereto have been cured or waived. None of Parent, Merger Sub or any of their respective affiliates or assigns, the Depositary and Paying Agent, the Information Agent, or any other person will be under any duty to give notification of any defects or irregularities in tenders or incur any liability for failure to give any such notification. Purchaser’s interpretation of the terms and conditions of the Offer (including the Letter of Transmittal and the instructions thereto and any other documents related to the Offer) will be final and binding, subject to the rights of the tendering holders of Shares to challenge our determination in a court of competent jurisdiction.

Except as otherwise provided in this Section 4, tenders of Shares pursuant to the Offer are irrevocable. However, a stockholder has withdrawal rights that are exercisable until the expiration of the Offer (i.e., at any time prior to one minute after 11:59 p.m. Eastern Time on August 13, 2025), or in the event the Offer is further extended, on such date and time to which the Offer is extended. In addition, pursuant to Section 14(d)(5) of the Exchange Act, Shares may be withdrawn at any time after September 14, 2025, which is the 60^th day after the date of the commencement of the Offer, unless prior to that date Purchaser has accepted for payment the Shares validly tendered in the Offer.

For a withdrawal of Shares to be effective, a written transmission notice of withdrawal must be timely received by the Depositary and Paying Agent at one of its addresses set forth on the back cover of this Offer to Purchase. Any notice of withdrawal must specify the name of the person having tendered the Shares to be

withdrawn, the number of Shares to be withdrawn and the name of the record holder of the Shares to be withdrawn, if different from that of the person who tendered such Shares. The signature(s) on the notice of withdrawal must be guaranteed by an Eligible Institution, unless such Shares have been tendered for the account of any Eligible Institution. If Shares have been tendered pursuant to the procedures for book-entry transfer as set forth in “The Tender Offer—Section 3. Procedures for Tendering Shares,” your nominee must submit your withdrawal through DTC to be credited with the withdrawn Shares. If certificates representing the Shares have been delivered or otherwise identified to the Depositary and Paying Agent, the name of the registered owner and the serial numbers shown on such certificates must also be furnished to the Depositary and Paying Agent prior to the physical release of such certificates.

All questions as to the form and validity (including time of receipt) of any notice of withdrawal will be determined by us, in our sole discretion (which may delegate power in whole or in part to the Paying and Depositary Agent), which determination will be final and binding, subject to the rights of the tendering holders of Shares to challenge our determination in a court of competent jurisdiction. No withdrawal of Shares will be deemed to have been properly made until all defects and irregularities have been cured or waived. None of Parent, Merger Sub or any of their respective affiliates or assigns, the Depositary and Paying Agent, the Information Agent, or any other person will be under any duty to give notification of any defects or irregularities in any notice of withdrawal or incur any liability for failure to give such notification. Withdrawals of tenders of Shares may not be rescinded, and any Shares properly withdrawn will be deemed not to have been validly tendered for purposes of the Offer. However, withdrawn Shares may be re-tendered by following one of the procedures for tendering Shares described in “The Tender Offer—Section 3. Procedures for Tendering Shares” at any time prior to the expiration of the Offer.

If Purchaser extends the Offer, delays its acceptance for payment of Shares, or is unable to accept for payment Shares pursuant to the Offer, for any reason, then, without prejudice to Purchaser’s rights pursuant to the Offer, the Depositary and Paying Agent may nevertheless, on Purchaser’s behalf, retain tendered Shares, and such Shares may not be withdrawn except to the extent that tendering stockholder’s exercise of withdrawal rights as described in this Section 4.

The following description of IGM and its business has been taken from IGM’s Annual Report on Form 10-K for the year ended December 31, 2024 filed with the SEC on March 6, 2025, the Amendment No. 1 to IGM’s Annual Report on Form 10-K filed with the SEC on April 30, 2025, IGM’s Quarterly Report on Form 10-Q for the quarter ended March 31, 2025 filed with the SEC on May 13, 2025, and IGM’s Current Reports on Form 8-K filed with the SEC on January 10, 2025, March 6, 2025, May 8, 2025, May 23, 2025, June 6, 2025 and July 1, 2025, and is qualified in its entirety by reference to such Form 10-K, Form 10-Q and Forms 8-K, respectively.

IGM is a biotechnology company that has focused on developing engineered IgM-based therapeutic antibodies.

IGM was incorporated under the laws of the State of Delaware on August 25, 1993 under the name Palingen, Inc. The name was changed to IGM Biosciences, Inc. on October 13, 2010. IGM terminated its lease agreements for office space and, accordingly, IGM does not maintain headquarters. For purposes of compliance with applicable requirements of the Securities Act of 1933, as amended, and Securities Exchange Act of 1934, as amended, any stockholder communication required to be sent to IGM’s principal executive offices may be sent to IGM Biosciences, Inc., 3 East Third Avenue, Suite 200, San Mateo, California 94401. IGM’s telephone number is (650) 965-7873.

Available Information. IGM is subject to the information and reporting requirements of the Exchange Act and in accordance therewith is obligated to file reports and other information with the SEC relating to its business, financial condition and other matters. Certain information, as of particular dates, concerning IGM’s business, principal physical properties, capital structure, material pending litigation, operating results, financial condition, directors and officers (including their remuneration and stock options granted to them), the principal holders of IGM’s securities, any material interests of such persons in transactions with IGM, and other matters is required to be disclosed in proxy statements and periodic reports distributed to IGM stockholders and filed with the SEC. The SEC maintains an Internet website that contains reports, proxy statements and other information about issuers, such as IGM, who file electronically with the SEC. The address of that site is https://www.sec.gov.

IGM also maintains an Internet website at www.igmbio.com. The information contained in, accessible from or connected to IGM’s website is not incorporated into, or otherwise a part of, this Offer to Purchase or any of IGM’s filings with the SEC. The website addresses referred to in this paragraph are inactive text references and are not intended to be actual links to the websites.

Sources of Information. Except as otherwise set forth herein, the information concerning IGM contained in this Offer to Purchase has been based upon publicly available documents and records on file with the SEC, other public sources and information provided by IGM. Although we have no knowledge that any such information contains any misstatements or omissions, none of the Guarantors, Parent, Merger Sub or any of their respective affiliates or assigns, the Information Agent or the Depositary and Paying Agent assumes responsibility for the accuracy or completeness of the information concerning IGM contained in such documents and records or for any failure by IGM to disclose events which may have occurred or may affect the significance or accuracy of any such information.

General. Parent is a Delaware limited liability company formed under the laws of the State of Delaware on March 8, 2023 with its principal offices located at 4747 Executive Drive, Suite 210, San Diego, California 92121. The telephone number of Parent is (858) 281-5372.

Merger Sub is a Delaware corporation with its principal offices located at 4747 Executive Drive, Suite 210, San Diego, California 92121. The telephone number of Merger Sub is (858) 281-5372. Merger Sub is a wholly owned subsidiary of Parent. Merger Sub has not engaged, and does not expect to engage, in any business other than in connection with the Offer and the Merger.

TCP is a Delaware limited partnership with its principal offices located at 4747 Executive Drive, Suite 210, San Diego, California 92121. The telephone number of TCP is (858) 200-3830.

TCM is the general partner of TCP and sole manager of Parent. TCM is a Delaware limited liability company with its principal offices located at 4747 Executive Drive, Suite 210, San Diego, California 92121. The telephone number of TCM is (858) 200-3830.

The name, citizenship, business address, business phone number, present principal occupation or employment and past material occupation, positions, offices or employment for at least the last five years for each director and each of the executive officers of Parent, Merger Sub, TCP and TCM (the “Item 3 Persons”) and certain other information are set forth in Schedule A hereto.

During the last five (5) years, none of Parent, Merger Sub, TCP or TCM or, to the knowledge of Parent, Merger Sub and the Guarantors, any of the Item 3 Persons: (i) has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors); or (ii) was a party to any judicial or administrative proceeding (except for matters that were dismissed without sanction or settlement and convictions that have been overturned on appeal) that resulted in a judgment, decree or final order enjoining the person from future violations of, or prohibiting activities subject to, federal or state securities laws, or a finding of any violation of such laws.

In the past sixty (60) days, TCP has not effected any transaction in the IGM Shares. Except as set forth in the preceding sentence or as otherwise described in this Offer to Purchase: (A) none of Parent, Merger Sub, TCP, TCM, any majority-owned subsidiary of Parent, Merger Sub, TCP or TCM or, to the knowledge of Parent, Merger Sub, TCP or TCM, any of the Item 3 Persons or any associate of any of the persons so listed beneficially owns or has any right to acquire, directly or indirectly, any Shares; and (B) none of Parent, Merger Sub, TCP or TCM or, to the knowledge of Parent, Merger Sub, TCP or TCM, any of the persons or entities referred to in clause (A) above nor any director, executive officer or subsidiary of any of the foregoing has effected any transaction in the Shares during the past 60 days. As of the date of this Offer to Purchase, TCP does not own any of the outstanding IGM Shares. As discussed in Section 10 – “Background of the Offer; Contacts with IGM,” any Shares owned by directly or indirectly by Parent or Merger Sub as of immediately prior to the Effective Time will be cancelled in the Merger for no consideration (including that no CVRs will be issued in respect of such Shares). There are no restrictions on any IGM stockholder with respect to transferring or disposing of any such Shares prior to the Effective Time.

Except as otherwise described in this Offer to Purchase, none of Parent, Merger Sub or TCP or, to the knowledge of Parent, Merger Sub, TCP or TCM, any of the Item 3 Persons, has any contract, arrangement,

understanding or relationship with any other person with respect to any securities of IGM, including, but not limited to, any contract, arrangement, understanding or relationship concerning the transfer or voting of such securities, finder’s fees, joint ventures, loan or option arrangements, puts or calls, guarantees of loans, guarantees against loss, guarantees of profits, division of profits or loss or the giving or withholding of proxies.

Except as set forth in this Offer to Purchase, none of Parent, Merger Sub, TCP, TCM or, to the knowledge of Parent, Merger Sub, TCP or TCM, any of the Item 3 Persons, has had any business relationship or transaction with IGM or any of its executive officers, directors or affiliates that is required to be reported under the rules and regulations of the SEC applicable to the Offer. Except as set forth in this Offer to Purchase, there have been no contacts, negotiations or transactions between Parent, Merger Sub, TCP or TCM or any of their subsidiaries or, to the knowledge of Parent, Merger Sub, TCP, TCM, any of the Item 3 Persons, on the one hand, and IGM or its affiliates, on the other hand, concerning a merger, consolidation or acquisition, tender offer or other acquisition of securities, an election of directors or a sale or other transfer of a material amount of assets during the past two years.

Available Information. Pursuant to Rule 14d-3 under the Exchange Act, Parent filed with the SEC a Tender Offer Statement on Schedule TO (the “Schedule TO”), of which this Offer to Purchase forms a part, and exhibits to the Schedule TO. Copies of the Schedule TO and the exhibits thereto, and reports, proxy statements and other information may be obtained by mail, upon payment of the SEC’s customary charges, by writing to its principal office at 100 F Street, NE, Washington, DC 20549. The Schedule TO and the exhibits thereto, as well as other information filed by Parent with the SEC, are available at the SEC’s website on the Internet at www.sec.gov that contains the Schedule TO and the exhibits thereto and other information that Purchaser has filed electronically with the SEC.

The following summary of certain provisions of the Merger Agreement and all other provisions of the Merger Agreement discussed herein are qualified by reference to the Merger Agreement itself, which is incorporated herein by reference. The Merger Agreement was filed as Exhibit 2.1 to the Current Report on Form 8-K that IGM filed with the SEC on July 1, 2025. The Merger Agreement may be examined and copies may be obtained at the places and in the manner set forth in “The Tender Offer—Section 6. Certain Information Concerning Parent and Merger Sub.” Stockholders and other interested parties should read the Merger Agreement for a more complete description of the provisions summarized below. Capitalized terms used herein and not otherwise defined have the respective meanings set forth in the Merger Agreement.

The Merger Agreement has been included to provide investors and security holders with information regarding its terms. It is not intended to provide any other factual information about IGM, Parent, Merger Sub or their respective subsidiaries and affiliates. The Merger Agreement contains representations and warranties by IGM, on the one hand, and Parent and Merger Sub, on the other hand, made solely for the benefit of the other. The assertions embodied in those representations and warranties are subject to qualifications and limitations agreed to by the respective parties in negotiating the terms of the Merger Agreement, including information in confidential disclosure schedules delivered in connection with the signing of the Merger Agreement. Moreover, certain representations and warranties in the Merger Agreement were made as of a specified date, may be subject to a contractual standard of materiality different from what might be viewed as material to investors, or may have been used for the purpose of allocating risk between IGM, on the one hand, and Parent and Merger Sub, on the other hand, rather than establishing matters as facts. Accordingly, the representations and warranties in the Merger Agreement should not be relied on by any persons as characterizations of the actual state of facts about IGM, Parent, Merger Sub or their respective subsidiaries or affiliates at the time they were made or otherwise. In addition, information concerning the subject matter of the representations and warranties may change after the date of the Merger Agreement, which subsequent information may or may not be fully reflected in IGM’s public disclosures.

The Offer. The Merger Agreement provides that Purchaser will commence the Offer no later than July 16, 2025. Purchaser’s obligation to accept for payment and pay for Shares validly tendered in the Offer is subject only to the satisfaction of the Minimum Tender Condition and the other Offer Conditions that are described in “The Tender Offer—Section 9. Conditions of the Offer.” Subject to the satisfaction of the Minimum Tender Condition and the other Offer Conditions that are described in “The Tender Offer—Section 9. Conditions of the

Offer,” the Merger Agreement provides that Purchaser will promptly after the applicable Expiration Date, as it may be extended pursuant to the terms of the Merger Agreement, irrevocably accept for payment and pay for all Shares validly tendered and not properly withdrawn pursuant to the Offer. Parent is entitled to use or cause to be used some or all of the Closing Net Cash in order to pay the Offer Price for all such Shares.

Purchaser expressly reserves the right, in its sole discretion, to: (i) waive, in whole or in part, any Offer Condition described in “The Tender Offer—Section 9. Conditions of the Offer” (including the Minimum Cash Condition), other than the Minimum Tender Condition or the Termination Condition; and/or (ii) modify the terms of the Offer in a manner not inconsistent with the Merger Agreement, except that without IGM’s prior written consent, Purchaser shall not:

The Merger Agreement provides that, unless the Merger Agreement has been validly terminated in accordance with its terms, Purchaser may elect to (and if so requested by IGM, Purchaser will) extend the Offer: (A) for one or more consecutive increments of such duration as requested by IGM (or if not requested by IGM, as determined by Parent), but not more than 10 business days each (or for such longer period as may be agreed to by Parent and IGM), if at the scheduled Expiration Date of the Offer (i) any of the Offer Conditions (as set forth in “The Tender Offer—Section 9. Conditions of the Offer”) shall not have been satisfied or waived, until such time as such conditions shall have been satisfied or waived or (ii) the determination of the Closing Net Cash and resolution of any dispute thereof has not been finalized in accordance with the terms of the Merger Agreement, and (B) Purchaser shall extend the Offer for the minimum period required by any rule, regulation or interpretation or position of the SEC or the staff thereof or Nasdaq applicable to the Offer; provided that Purchaser not be permitted or required to extend the Offer beyond the Outside Date.

Unless the Merger Agreement is terminated in accordance with its terms, Purchaser shall not terminate or withdraw the Offer prior to any scheduled expiration date. In the event the Merger Agreement is validly terminated in accordance with its terms, Purchaser will promptly and irrevocably terminate the Offer and return and will cause any depository acting on behalf of Purchaser to return, all tendered Shares to the registered holders thereof.

Conversion of Capital Stock at the Effective Time. Each outstanding Share, other than Shares held in the treasury by IGM, held by any stockholders, or owned by any beneficial owners, of IGM who are entitled to and who properly exercise appraisal rights under Delaware law or owned by Parent, Merger Sub or any other subsidiary of Parent at the commencement of the Offer and is owned by Parent, Merger Sub or any other subsidiary of Parent immediately prior to the Effective Time, will be converted into the right to receive the Offer Price, without interest and subject to any applicable tax withholding.

Each share of Merger Sub’s capital stock issued and outstanding immediately prior to the Effective Time will be converted into and become one fully paid and nonassessable share of common stock, par value $0.01 per share, of the Surviving Corporation.

The Merger. The Merger Agreement provides that, as soon as practicable following the Offer Closing Time and subject to the terms and conditions of the Merger Agreement, and in accordance with the DGCL, at the Effective Time, Merger Sub will be merged with and into IGM, the separate existence of Merger Sub will cease and IGM will continue as the Surviving Corporation in the Merger. The Merger will be effected under Section 251(h) of the DGCL, without a vote on the adoption of the Merger Agreement by the holders of the Shares. Accordingly, Parent, Merger Sub and IGM have agreed to take all necessary and appropriate action to cause the Merger to become effective as soon as practicable following the consummation of the Offer without a vote of IGM stockholders in accordance with Section 251(h) of the DGCL, upon the terms and subject to the satisfaction or waiver of the conditions to the Merger.

Immediately following the Effective Time, the certificate of incorporation of IGM will be amended and restated in its entirety to be in the form attached as Exhibit B to the Merger Agreement and, as so amended and restated, will be the certificate of incorporation of the Surviving Corporation.

Immediately following the Effective Time, the bylaws of IGM will be amended and restated in its entirety to be in the form attached as Exhibit C to the Merger Agreement and, as so amended and restated, will be the bylaws of the Surviving Corporation.

Treatment of Equity Awards. Pursuant to the Merger Agreement, as of immediately prior to the Effective Time, the vesting for each Company Stock Option and, as of immediately prior to the Offer Closing Time, the vesting for each Company Restricted Stock Unit, shall be accelerated and: (i) (A) each Company Stock Option that has an exercise price per share that is less than the Cash Amount (each, an “In-the-Money Option”) that is then outstanding will be cancelled and, in exchange therefor, the holder of such cancelled In-the-Money Option will be entitled to receive in consideration of the cancellation of such In-the-Money Option (1) an amount in cash without interest, less any applicable tax withholding, equal to the product obtained by multiplying (x) the excess of the Cash Amount over the exercise price per share of Shares underlying such In-the-Money Option by (y) the number of Shares underlying such In-the-Money Option as of immediately prior to the Effective Time, and (2) one CVR for each Share subject thereto, and (B) each Company Stock Option that has an exercise price per share that is equal to or greater than the Cash Amount that is then outstanding will be cancelled for no consideration; and (ii) each Company Restricted Stock Unit that is then outstanding will be cancelled and, in exchange therefor, the holder of such cancelled Company Restricted Stock Unit will be entitled to receive in consideration of the cancellation of such Company Restricted Stock Unit an (1) amount in cash without interest, less any applicable tax withholding, equal to the Cash Amount, and (B) one CVR.

Treatment of Company Pre-Funded Warrants. Prior to the Effective Time, IGM shall satisfy all of its notification requirements under the terms of each outstanding and unexercised Company Pre-Funded Warrant, and, at the Effective Time, each Company Pre-Funded Warrant shall be canceled and holders of such warrants will be entitled to receive: (1) an amount in cash without interest, less any applicable tax withholding, equal to the product obtained by multiplying (x) the excess of the Cash Amount over the exercise price per share of such Company Pre-Funded Warrant by (y) the number of Shares underlying such Company Pre-Funded Warrant as of immediately prior to the Effective Time; and (2) one CVR for each Share underlying such Company Pre-Funded Warrant.

Conditions to Each Party’s Obligation to Effect the Merger. The obligation of IGM, Parent and Merger Sub to effect the Merger is subject to the satisfaction or waiver on or prior to the Closing Date of the following conditions:

IGM Board Recommendation. As described above, and subject to the provisions described below, the IGM Board has determined to recommend that the stockholders of IGM accept the Offer and tender their Shares to

Purchaser in the Offer. The foregoing recommendation is referred to herein as the “IGM Board Recommendation.” The IGM Board also agreed to include the IGM Board Recommendation with respect to the Offer in the Schedule 14D-9 and has permitted Parent to refer to such recommendation in this Offer to Purchase and documents related to the Offer.

Reasonable Best Efforts. Each of IGM, Parent and Merger Sub has agreed to use its respective reasonable best efforts to promptly take, or cause to be taken, and to do, or cause to be done, and to assist and cooperate with the other parties to the Merger Agreement in doing, all actions that are necessary, proper or advisable to consummate and make effective, as promptly as reasonably practicable and in any event prior to the Outside Date, the other transactions contemplated by the Merger Agreement (collectively, the “Transactions”). In particular, without limiting the generality of the foregoing, IGM, Parent and Merger Sub are required to use reasonable best efforts to: (i) cause each of the Offer Conditions and each of the conditions to the Merger to be satisfied as promptly as reasonably practicable; (ii) obtain all necessary or advisable actions or non-actions, waivers and consents from, or the making of all necessary registrations, declarations and filings with, and the taking of all reasonable steps as may be necessary to avoid a proceeding by, any governmental entity with respect to the Merger Agreement or the Transactions; (iii) defending or contesting of any proceedings, whether judicial or administrative, challenging the Merger Agreement or the consummation of the Transactions, including seeking to have any stay or temporary restraining order entered by any court or other governmental entity vacated or reversed; and (iv) executing and delivering any additional instruments necessary to consummate the Transactions and to fully carry out the purposes of the Merger Agreement.

Termination. The Merger Agreement may be terminated prior to the Offer Closing Time as follows:

Termination Fee. IGM has agreed to pay Parent a termination fee of $2,425,000 (the “Company Termination Fee”) if:

IGM is obligated to pay to Parent an expense reimbursement payment equal to the reasonable and documented out-of-pocket fees and expenses incurred by or on behalf of Parent or its affiliates in connection with the Transactions and CVR Agreement up to a maximum of $500,000 if Parent terminates the Merger Agreement pursuant to a Minimum Cash Termination (the “Expense Reimbursement Payment”).

In the event Parent receives the Company Termination Fee or Expense Reimbursement Payment, such receipt will be deemed to be liquidated damages for any and all losses or damages suffered or incurred by Parent or Merger Sub and constitute their sole and exclusive remedy against IGM and its current, former or future stockholders and representatives for any loss suffered as a result of the failure of the Transactions to be consummated, and none of IGM and its current, former or future stockholders or representatives will have any further liability or obligation relating to or arising out of the Merger Agreement or the Transactions, except as set forth in the Merger Agreement.

Effect of Termination. If the Merger Agreement is terminated pursuant to its terms, the Merger Agreement will become void and have no effect and there will be no liability or obligation on the part of Parent, Merger Sub or IGM following any such termination, except that: (i) certain specified provisions of the Merger Agreement will survive, including those described in “Termination Fee” above; (ii) the confidentiality agreement by and among Parent and IGM will survive the termination of the Merger Agreement and remain in full force and effect in accordance with its terms; and (iii) termination will not relieve any party from liability for fraud or any willful and material breach of such party’s representations, warranties, covenants or agreements set forth in the Merger Agreement.

Conduct of Business Pending the Merger. IGM has agreed that, from the date of the Merger Agreement until the earlier of the Offer Closing Time and the termination of the Merger Agreement in accordance with its terms, except as specifically provided by the Merger Agreement or as consented to in writing in advance by Parent, IGM shall use commercially reasonable efforts to carry on its business in a manner consistent with the Wind-Down Process (as defined in the Merger Agreement) and otherwise in the ordinary course of business. In addition, except as set forth in the IGM Disclosure Letter or otherwise expressly and specifically permitted or required by the Merger Agreement or applicable law, from the date of the Merger Agreement until the earlier of the Offer Closing Time and the termination of the Merger Agreement in accordance with its terms, IGM shall not do any of the following without the prior written consent of Parent (which shall not be unreasonably withheld, delayed or conditioned):

Access to Information. Except if prohibited by applicable law, during the period prior to the earlier of the Effective Time or the termination of the Merger Agreement in accordance with its terms, IGM will provide Parent and its Representatives reasonable access during normal business hours (under supervision of appropriate personnel and in a manner that does not unreasonably interfere with the normal operation of IGM’s business) to its properties, books and records, contracts and personnel, and will furnish to Parent such information concerning IGM’s business, properties and personnel as Parent or its representatives may reasonably request.

Security Holder Litigation. In the event that any litigation commences or is threatened in writing by or on behalf of one or more stockholders of IGM against IGM and its directors relating to any Transaction, IGM has agreed to provide Parent an opportunity to review and propose comments to all material filings or responses to such litigation. Parent’s consent is required for IGM to enter into, agree to or disclose any settlement with respect to any such litigation, except to the extent such settlement is covered by IGM’s insurance policies or

relates to the provision of additional disclosure in the Schedule 14D-9, but in each case, only if such settlement would not result in any restriction on the business or operations of IGM or its affiliates. IGM has an obligation to notify Parent of the commencement or written threat of any litigation and to keep Parent promptly and reasonably informed regarding any such litigation.

Indemnification, Exculpation and Insurance. All rights to indemnification and exculpation from liabilities for acts or omissions occurring at or prior to the Effective Time, and rights to advancement of expenses, existing in favor of any person who is, becomes, or has ever been, a director, officer, employee or agent (including as a fiduciary with respect to an employee benefit plan) of IGM or its predecessors (each, an “Indemnified Party”), in each case, as provided in IGM’s charter, bylaws or any indemnification agreement between IGM and an Indemnified Party: (i) will be assumed by the Surviving Corporation at the Effective Time; (ii) will survive the Merger; (iii) will continue in full force and effect in accordance with their terms; and (iv) for a period of six years following the date of the Merger Agreement, will not be amended, repealed or otherwise modified in any manner adverse to such Indemnified Party. Parent has agreed to ensure the Surviving Corporation complies with the foregoing obligations.

Stock Exchange Delisting and Deregistration. As promptly as practicable following the Effective Time, the Surviving Corporation will cause IGM’s securities to be de-listed from Nasdaq and de-registered under the Exchange Act.

Section 16 Matters. Prior to the Effective Time, Parent will, and IGM may, take all steps as may be required to cause any dispositions or cancellations of IGM’s equity securities in connection with the Merger Agreement or the Transactions by each individual who is a director or officer of IGM subject to Section 16 of the Exchange Act to be exempt under Rule 16b-3 under the Exchange Act.

Takeover Laws. Parent, IGM and the IGM Board have agreed to: (i) take all actions within their power to ensure that no “business combination,” “control share acquisition,” “fair price,” “moratorium” or other anti-takeover law (each, a “Takeover Law”) is or becomes applicable to the Merger Agreement, the Offer, Offer Documents, the Merger or any of the other Transactions; and (ii) if any Takeover Law becomes applicable to the Merger Agreement, the Offer, Offer Documents, the Merger or any of the other Transactions, take all action within their power to ensure that the Offer, the Merger and the other Transactions may be consummated as promptly as practicable on the terms contemplated by the Merger Agreement and otherwise to minimize the effect of such statute or regulation on the Offer, the Merger and the other Transactions.

Governance of the Surviving Corporation. Immediately following the Effective Time: (i) the directors of the Merger Sub immediately prior to the Effective Time will be appointed as the directors of the Surviving Corporation; and (ii) the officers of Merger Sub immediately prior to the Effective Time will become the officers of the Surviving Corporation.

Public Announcements. Parent, Merger Sub and IGM have agreed to consult with each other before issuing, and provide each other the opportunity to review and comment upon, any press release or other public statements with respect to the Offer, the Merger and the other Transactions, and shall not issue any such press release or make any such public statement prior to such consultation, except as may be required by applicable law, court process or by obligations pursuant to any listing agreement with any national or foreign securities exchange.

Representations and Warranties. This summary of the Merger Agreement has been included to provide investors with information regarding its terms. It is not intended to provide any other factual information about Parent, Merger Sub or IGM, their respective businesses, or the actual conduct of their respective businesses during the period prior to the consummation of the Offer or the Merger. The Merger Agreement contains representations and warranties that are the product of negotiations among the parties thereto and made to, and solely for the benefit of, each other as of the specified dates therein. The assertations embodied in those representations and warranties are subject to qualifications and limitations agreed to by the respective parties and are also qualified in important part by confidential disclosure scheduled delivered by IGM to Parent in connection with the Merger Agreement.

The representations and warranties were negotiated with the principal purpose of allocating risk among the parties to the agreements instead of establishing these matters as facts, and may be subject to standards of materiality applicable to the contracting parties that differ from those applicable to investors.

In the Merger Agreement, IGM has made representations and warranties to Parent and Merger Sub with respect to, among other things:

Some of the representations and warranties in the Merger Agreement made by IGM are qualified as to “materiality” or a “Company Material Adverse Effect.” For purposes of the Merger Agreement, a “Company Material Adverse Effect” means any change, event, condition, development, circumstance, state of facts, effect or occurrence that has a material adverse effect on (i) the business, financial condition, assets, properties or results of operations of IGM or (ii) the ability of IGM to consummate the Transactions. Solely with respect to the foregoing clause (i) of the definition of Company Material Adverse Effect, a Company Material Adverse Effect does not include or take into account any change, event, condition, development, circumstance, state of facts, effect or occurrence to the extent resulting from or arising out of:

In the Merger Agreement, Parent and Merger Sub have made representations and warranties to IGM with respect to:

Some of the representations and warranties in the Merger Agreement made by Parent and Merger Sub are qualified as to “materiality” or a “Parent Material Adverse Effect.” For purposes of the Merger Agreement, a “Parent Material Adverse Effect” means any change, effect, event or occurrence that prevents Parent or Merger Sub from consummating the Offer, the Merger and the other Transactions on or before the Outside Date.

None of the representations, warranties, covenants or agreements in the Merger Agreement or in any instrument delivered pursuant to the Merger Agreement will survive the Effective Time, other than those covenants or agreements of the parties which by their terms apply, or are to be performed in whole or in part, after the Effective Time.

Specific Performance. The parties have agreed that irreparable damage would occur in the event that any of the provisions of the Merger Agreement were not performed in accordance with their specific terms or were otherwise breached, and that monetary damages, even if available, would not be an adequate remedy therefor. The parties further agreed that the parties will be entitled to an injunction or injunctions, or any other appropriate form of equitable relief, to prevent breaches of the Merger Agreement and to enforce specifically the performance of the terms and provisions of the Merger Agreement in Chancery Court of the State of Delaware (or in any federal court located in the State of Delaware if jurisdiction is not then available in the Chancery Court of the State of Delaware) in addition to any other remedy to which they are entitled at law or in equity.

Expenses. Except as otherwise provided in the Merger Agreement, all fees and expenses incurred by the parties in connection with the Merger Agreement and the Transactions will be paid by the party incurring such expenses, whether or not the Offer or the Merger is consummated.

Offer Conditions. The Offer Conditions are described in “The Tender Offer—Section 9. Conditions of the Offer.”

CVR Agreement. At or prior to the Offer Closing Time, Parent, Merger Sub, the Rights Agent (the “Rights Agent”) and the Representative of the holders of the CVRs (the “Representative”) will enter into the CVR Agreement governing the terms of certain contingent consideration payable thereunder. Each CVR will represent a contractual right to receive certain contingent cash payments equal to a pro rata share of the following, each pursuant to the CVR Agreement and collectively, “CVR Proceeds”:

The CVRs are not deferred cash payments to IGM stockholders but instead are contractual rights to receive one or more payments in cash, contingent upon certain events occurring (as described above).

IGM stockholders will be paid promptly in compliance as described in the CVR Agreement. The rights of holders of CVRs will be fixed under the CVR Agreement. The CVRs represent the binding obligations of Parent under the CVR Agreement, and pursuant to Section 10.05 of the Merger Agreement, the holders of CVRs are intended third-party beneficiaries of the rights expressly provided for their benefit in the CVR Agreement.

You should also understand that, as discussed above, the co-offerors estimate that the amount that will be payable under the CVRs is most likely between $0.00–$0.02 per CVR, consisting of: (i) $0.00 per CVR in Disposition Proceeds; and (ii) $0.00–$0.02 per CVR in Additional Closing Net Cash Proceeds. However, this range of values is based on estimates and assumptions regarding future events, as more fully described below, and there is no guarantee

that any payment will be made to holders of the CVRs, and, thus, for purposes of determining whether to tender your Shares in the Offer you should assume that no payment will be made under the CVR Agreement and the only consideration paid by the co-offerors will be the Cash Amount of $1.247 per Share.

The CVR Agreement provides the holders of CVRs certain contractual rights against Parent, Merger Sub and the Rights Agent. The consent of the holders of at least 40% of the outstanding CVRs (the “Acting Holders”) is required any amendments to the CVR Agreement for the purpose of adding, eliminating, or changing provisions therein. The Acting Holders also have the right, on behalf of all holders of CVRs, to institute any action or proceeding at law, in equity, in bankruptcy or otherwise with respect to CVR Agreement, though no individual holder will be entitled to exercise such rights independently. Furthermore, the consent of the Representative is required for Parent or its affiliates to enter into any Disposition Agreement or other agreement providing for a change of control with Parent or its affiliates as the acquiror, licensee or recipient of any CVR Product.

The right to payments under the CVRs as evidenced by the CVR Agreement is a contractual right only and will not be transferable, except in the limited circumstances specified in the CVR Agreement, including: (a) upon death of a Holder by will or intestacy; (b) pursuant to a court order; (c) by operation of law (including by consolidation or merger) or without consideration in connection with the dissolution, liquidation or termination of any corporation, limited liability company, partnership or other entity; (d) in the case of CVRs held in book-entry or other similar nominee form, from a nominee to a beneficial owner and, if applicable, through an intermediary, to the extent allowable by DTC); or (e) that CVRs may be abandoned. The CVRs are not securities, and therefore will not be registered with the SEC.

The foregoing description of the CVR Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the form of the CVR Agreement, which is filed as Exhibit (d)(4) of the Schedule TO.

Confidentiality Agreement. IGM and Parent, entered into a confidentiality agreement (the “Confidentiality Agreement”) dated May 27, 2025, pursuant to which Parent and its affiliates, including TCM and TCP, agreed, subject to certain exceptions, to keep confidential all proprietary, nonpublic and/or confidential information about the other party, its affiliates or subsidiaries and/or its business furnished in connection with a possible negotiated transaction. Parent’s and TCM’s obligations under the Confidentiality Agreement will expire three (3) years after the date of the Confidentiality Agreement. The Confidentiality Agreement included customary standstill and fall-away provisions.

This summary and description of the material terms of the Confidentiality Agreement does not purport to be complete and is qualified in its entirety by reference to the Confidentiality Agreement, which is filed as Exhibit (d)(2) to the Schedule TO and is incorporated by reference herein.

Limited Guaranty. Concurrently with the execution of the Merger Agreement, and as a condition and inducement to IGM’s willingness to enter into the Merger Agreement, Tang Capital Partners, LP, a Delaware limited partnership (“TCP” or “Guarantor”) and sole member of Parent, delivered to IGM a duly executed limited guaranty (the “Limited Guaranty”), dated as of the date of the Merger Agreement, in favor of IGM, in respect of certain obligations of Parent and Merger Sub under the Merger Agreement and the CVR Agreement. Certain obligations under the Limited Guaranty are subject to a cap of $78,000,000, which includes certain Enforcement Costs, under the Merger Agreement and an amount equivalent to the CVR Proceeds (as defined below) under the CVR Agreement.

The Offer is not conditioned upon Parent’s, Merger Sub’s or the Guarantor’s ability to finance the purchase of Shares pursuant to the Offer. Parent and Merger Sub estimate that the total amount of funds required to consummate the Merger (including payments for the settlement and cancellation of Company Stock Options and Company Restricted Stock Units) pursuant to the Merger Agreement and to purchase all of the Shares pursuant to the Offer and the Merger Agreement is approximately $78 million, excluding any CVR Proceeds. The funds to pay for all Shares accepted for payment in the Offer may be funded entirely with available cash at Parent. Payments of CVR Proceeds, if any, will be self-funded with: (A) the Net Proceeds from the applicable sale, transfer, license or other disposition or grant of rights by Parent or any of its affiliates, including IGM after the Merger, of all or any part of the CVR Products or the Company Patents that occurs within the period beginning

on the Merger Closing Date and ending on the first (1^st) anniversary following the Merger Closing Date, and such Gross Proceeds are received by or otherwise due to Parent or any of its Affiliates by the first (1^st) anniversary following the Merger Closing Date, if any; and/or (B) with net savings versus the Closing Net Cash that are realized prior to the Merger Closing Date, adjusted for any claims Parent reasonably determines to be valid five (5) business days prior to the Merger Closing Date that are not accounted for in such Closing Net Cash, if any.

In connection with the execution of the Merger Agreement, the Guarantor has agreed to the Limited Guaranty. Subject to the terms of the Limited Guaranty, the Guarantor has agreed to guarantee certain obligations of Parent and Merger Sub up to $78,000,000, which includes certain Enforcement Costs, under the Merger Agreement and up to amount equivalent to the CVR Proceeds under the CVR Agreement. Other than the Limited Guaranty, there are no financing arrangements or alternative financing plans.

We do not believe our financial condition is relevant to your decision whether to tender your Shares and accept the Offer because: (i) the Offer is being made for all outstanding Shares solely for cash and CVRs (which will be self-funded, as described above); (ii) the Offer is not subject to any financing condition; (iii) if we consummate the Offer, we will acquire all remaining Shares for the same price in the Merger; and (iv) the Guarantor has agreed to guarantee the obligations of Parent and Merger Sub under the Merger Agreement, subject to the terms and conditions set forth in the Limited Guaranty.

Notwithstanding any other term of the Offer or the Merger Agreement, Purchaser shall not be required to accept for payment or, subject to any applicable rules and regulations of the SEC, including Rule 14e-1(c) under the Exchange Act (relating to Purchaser’s obligation to pay for or return tendered Shares promptly after the termination or withdrawal of the Offer), to pay for any Shares tendered pursuant to the Offer and, subject to the terms of the Merger Agreement, may delay the acceptance for payment of or payment for Shares or may terminate or amend the Offer, if:

The foregoing conditions shall be in addition to, and not a limitation of, the rights of Purchaser to extend, terminate or modify the Offer and are described herein. The foregoing conditions are for the sole benefit of Purchaser and, subject to the terms and conditions of the Merger Agreement and the applicable rules and regulations of the SEC, may be waived by Purchaser in whole or in part at any time and from time to time in its sole discretion. Such rights of termination are described above in “The Tender Offer—Section 7. Summary of the Merger Agreement and Certain Other Agreements—Summary of the Merger Agreement—Termination.” All conditions (including the Minimum Cash Condition), other than the Minimum Tender Condition and the Termination Condition may be waived by Purchaser in its sole discretion in whole or in part at any applicable time or from time to time, in each case subject to the terms and conditions of the Merger Agreement and the applicable rules and regulations of the SEC. The failure by Parent, Merger Sub or any other Affiliate of Parent at any time to exercise any of the foregoing rights shall not be deemed a waiver of any such right, the waiver of any such right with respect to particular facts and circumstances shall not be deemed a waiver with respect to any other facts and circumstances and each such right shall be deemed an ongoing right that may be asserted at any time and from time to time. In accordance with SEC rules and regulations, upon discovery of a condition that gives rise to termination of the Offer, Parent and Merger Sub will undertake to promptly notify IGM stockholders of a decision to either terminate the Offer, or to waive the condition and proceed with the Offer.

The Merger Agreement provides that IGM will not (subject to certain exceptions), between the date of the Merger Agreement and the Effective Time, declare, set aside or pay any dividends on, or make any other distributions (whether in cash, stock or property) in respect of, any of its capital stock (including the Shares) or other equity interests, except for dividends by a wholly owned subsidiary of IGM to its parent. See “Special Factors—Section 4. Price Range of Shares; Dividends” and “The Tender Offer—Section 7. Summary of the Merger Agreement and Certain Other Agreements—Conduct of Business Pending the Merger.”

General. Except as otherwise set forth in this Offer to Purchase, based on Parent’s and Merger Sub’s review of publicly available filings by IGM with the SEC and other information regarding IGM, Parent and Merger Sub are not aware of any licenses or other regulatory permits which appear to be material to the business of IGM and which might be adversely affected by the acquisition of Shares by Purchaser pursuant to the Offer or of any approval or other action by any governmental, administrative or regulatory agency or authority which would be required for the acquisition or ownership of Shares by Purchaser pursuant to the Offer. In addition, except as set forth below, Parent and Merger Sub are not aware of any filings, approvals or other actions by or with any governmental body or administrative or regulatory agency that would be required for Purchaser’s acquisition or ownership of the Shares. Should any such approval or other action be required, Parent and Merger Sub have agreed to use reasonable best efforts to, in the most expeditious manner practicable, obtain all necessary actions or nonactions, waivers, consents, approvals, orders and authorizations from governmental entities, make all necessary registrations, declarations and filings and make all commercially reasonable efforts to obtain an

approval or waiver from, or to avoid any action by, any governmental entity. The parties currently expect that such approval or action, except as described below under “Takeover Laws,” would be sought or taken. There can be no assurance that any such approval or action, if needed, would be obtained or, if obtained, that it will be obtained without substantial conditions; and there can be no assurance that, in the event that such approvals were not obtained or such other actions were not taken, adverse consequences might not result to IGM’s or Parent’s business or that certain parts of IGM’s or Parent’s business might not have to be disposed of or held separate. In such an event, we may not be required to purchase any Shares in the Offer. See “The Tender Offer—Section 9. Conditions of the Offer.”

Antitrust. Based on a review of the information currently available relating to the businesses in which Parent and IGM are engaged and the consideration to be paid for the Shares, Parent and Merger Sub have determined that no mandatory antitrust premerger notification filing or waiting period under Hart-Scott Rodino Antitrust Improvements Act of 1976, as amended (the “HSR”), and the rules and regulations promulgated thereunder is required, and therefore HSR clearance is not a condition to the consummation of the Offer or the Merger.

Based upon an examination of publicly available and other information relating to the businesses in which IGM is engaged, Parent and Merger Sub believe that the acquisition of Shares in the Offer (and the Merger) should not violate applicable antitrust laws. Nevertheless, Parent and Merger Sub cannot be certain that a challenge to the Offer (and the Merger) on antitrust grounds will not be made, or, if such challenge is made, what the result will be. See “The Tender Offer—Section 9. Conditions of the Offer.”

Stockholder Approval Not Required. Assuming the Offer and the Merger are consummated in accordance with Section 251(h) of the DGCL, IGM has represented in the Merger Agreement that execution, delivery and performance of the Merger Agreement by IGM and the consummation by IGM of the Offer and the Merger have been duly validly authorized by all necessary corporate action on the part of IGM, and no other corporate proceedings on the part of IGM are necessary to authorize the Merger Agreement or to consummate the Offer and the Merger. Section 251(h) of the DGCL provides that stockholder approval of a merger is not required if certain requirements are met, including that: (i) the acquiring company consummates an offer for all of the outstanding stock of the company to be acquired that, absent Section 251(h) of the DGCL, would be entitled to vote on such merger; (ii) following the consummation of such tender offer, the stock irrevocably accepted for purchase pursuant to the offer, together with the stock otherwise owned by the consummating company and its affiliates and any “rollover stock” (as defined in Section 251(h) of the DGCL), equals at least such percentage of the stock of the company to be acquired that, absent Section 251(h) of the DGCL, would be required to adopt the merger agreement; and (iii) the stockholders at the time of the merger receive the same consideration for their stock in the merger as was payable in the tender offer. If the Minimum Tender Condition is satisfied and we accept Shares for payment pursuant to the Offer, we will hold a sufficient number of Shares to ensure that IGM will not be required to submit the adoption of the Merger Agreement to a vote of its stockholders. Following the consummation of the Offer and subject to the satisfaction of the remaining conditions set forth in the Merger Agreement, Merger Sub, Parent and IGM will take all necessary and appropriate action to effect the Merger as promptly as practicable without a meeting of stockholders of IGM in accordance with Section 251(h) the DGCL. See “Special Factors—Section 2. Purpose of the Offer and Plans for IGM” and “The Tender Offer—Section 7. Summary of the Merger Agreement and Certain Other Agreements.”

Takeover Laws. A number of states (including Delaware, where IGM is incorporated) have adopted takeover laws and regulations which purport, to varying degrees, to be applicable to attempts to acquire securities of corporations which are incorporated in such states or which have substantial assets, stockholders, principal executive offices or principal places of business therein.

As a Delaware corporation, IGM has not opted out of Section 203 of the DGCL. In general, Section 203 of the DGCL would prevent an “interested stockholder” (generally defined in Section 203 of the DGCL as a person beneficially owning 15% or more of a corporation’s voting stock and the affiliates and associates of such person) from engaging in a “business combination” (as defined in Section 203 of the DGCL) with a Delaware corporation for three years following the time such person became an interested stockholder unless, among other exceptions: (i) before such person became an interested stockholder, the board of directors of the corporation approved the transaction in which the interested stockholder became an interested stockholder or approved the business combination; (ii) upon consummation of the transaction which resulted in the interested stockholder becoming an interested stockholder, the interested stockholder owned at least 85% of the voting stock of the

corporation outstanding at the time the transaction commenced (excluding for purposes of determining the number of shares of outstanding stock held by directors who are also officers and by employee stock plans that do not allow plan participants to determine confidentially whether to tender shares); or (iii) following the transaction in which such person became an interested stockholder, the business combination is: (A) approved by the board of directors of the corporation; and (B) authorized at a meeting of stockholders by the affirmative vote of the holders of at least 66 2/3% of the outstanding voting stock of the corporation not owned by the interested stockholder. The restrictions on business combinations contained in Section 203 of the DGCL also do not apply, among other possibilities, (i) to corporations that do not have a class of voting stock listed on a national securities exchange or held of record by more than 2,000 stockholders (unless the corporation’s certificate of incorporation expressly provides otherwise) or (ii) to interested stockholders who became interested stockholders at a time when the restrictions on business combinations did not apply because of the foregoing clause (i).

IGM has represented to us in the Merger Agreement, assuming certain of our representations as to our stock ownership in IGM are true, that the IGM Board has taken all actions so that the restrictions applicable to business combinations contained in Section 203 of the DGCL are not, and will not be, applicable to the execution, delivery or performance of the Merger Agreement and the timely consummation of the Offer and the Merger and will not restrict, impair or delay the ability of Parent or Merger Sub, after the Offer Closing Time, to vote or otherwise exercise all rights as a stockholder of IGM. Purchaser has not attempted to comply with any other state takeover statutes in connection with the Offer or the Merger. Purchaser reserves the right to challenge the validity or applicability of any state law allegedly applicable to the Offer, the Merger or the Merger Agreement, and nothing in this Offer to Purchase or any action taken in connection herewith is intended as a waiver of that right. In the event that it is asserted that one or more takeover statutes apply to the Offer, the Merger or the Merger Agreement, and it is not determined by an appropriate court that such statute or statutes do not apply or are invalid as applied to the Offer, the Merger or the Merger Agreement, as applicable, Purchaser may be required to file certain documents with, or receive approvals from, the relevant state authorities, and Purchaser might be unable to accept for payment or purchase Shares tendered pursuant to the Offer or be delayed in continuing or consummating the Offer. In such case, Purchaser may not be obligated to accept for purchase, or pay for, any Shares tendered. See “The Tender Offer—Section 9. Conditions of the Offer.”

Appraisal Rights. No appraisal rights are available to the holders of record or beneficial owners of Shares in connection with the Offer. However, if the Offer is successful and the Merger is consummated, stockholders of record or beneficial owners of Shares who: (i) did not tender their Shares in the Offer (or who had tendered but subsequently validly withdrawn such tender, and not otherwise waived their appraisal rights); (ii) otherwise comply with the applicable requirements and procedures of Section 262 of the DGCL; and (iii) do not thereafter withdraw their demand for appraisal of such Shares or otherwise lose their appraisal rights, in each case in accordance with the DGCL, will be entitled to demand appraisal of their Shares and receive in lieu of the consideration payable in the Merger a cash payment equal to the “fair value” of their Shares, as determined by the Delaware Court of Chancery, in accordance with Section 262 of the DGCL. If you choose to exercise your appraisal rights in connection with the Merger and you properly demand and perfect such rights in accordance with Section 262 of the DGCL, you may be entitled to payment for your Shares based on a judicial determination of the fair value of your Shares plus interest, if any, on the amount determined to be fair value.

The following discussion is not a complete statement of the law pertaining to appraisal rights under the DGCL and is qualified in its entirety by the full text of Section 262 of the DGCL, a copy of which may be accessed without subscription or cost at the following publicly available website: https://delcode.delaware.gov/title8/c001/sc09/index.html#262. All references in Section 262 of the DGCL and in this summary to a (i) “stockholder” are to the record holder of Shares unless otherwise expressly noted herein, (ii) “beneficial owner” are to a person who is the beneficial owner of Shares held either in voting trust or by a nominee on behalf of such person, and (iii) “person” are to an individual, corporation, partnership, unincorporated association or other entity. Stockholders and beneficial owners of Shares should carefully review the full text of Section 262 of the DGCL as well as the information discussed herein. Stockholders and beneficial owners of Shares should assume that IGM will take no action to perfect any appraisal rights of any person.

The “fair value” of the Shares as determined by the Delaware Court of Chancery could be based upon considerations other than, or in addition to, the price paid in the Offer and the Merger and the market value of such Shares. Stockholders and beneficial owners of Shares should recognize that the fair value determined in an appraisal proceeding of the Delaware Court of Chancery could be higher or lower than, or the same as, the Offer

Price and that an investment banking opinion as to the fairness, from a financial point of view, of the consideration payable in a sale transaction, such as the Offer and the Merger, is not an opinion as to, and does not otherwise address, fair value under the DGCL. Moreover, Parent and IGM may argue in an appraisal proceeding that, for purposes of such proceeding, the “fair value” of such Shares is less than the Offer Price.

Any person who desires to exercise his, her or its appraisal rights should review carefully Section 262 of the DGCL and is urged to consult his, her or its legal advisor before electing or attempting to exercise such rights.

Under Section 262 of the DGCL, if a merger is approved under Section 251(h) of the DGCL, either a constituent corporation before the effective date of the merger, or the surviving corporation within 10 days thereafter, must notify each of the holders of any class or series of stock of such constituent corporation who are entitled to appraisal rights of the approval of the merger and that appraisal rights are available for any or all shares of such class or series of stock of such constituent corporation, and shall include in such notice either a copy of Section 262 of the DGCL or information directing the stockholders to a publicly available electronic resource at which Section 262 of the DGCL may be accessed without subscription or cost. THE SCHEDULE 14D-9 CONSTITUTES THE FORMAL NOTICE OF APPRAISAL RIGHTS UNDER SECTION 262 OF THE DGCL. FAILURE TO FOLLOW THE STEPS REQUIRED BY SECTION 262 OF THE DGCL FOR EXERCISING AND PERFECTING APPRAISAL RIGHTS WILL RESULT IN THE LOSS OF SUCH RIGHTS.

As discussed in the Schedule 14D-9, stockholders and beneficial owners of Shares wishing to exercise the right to seek an appraisal of their Shares under Section 262 of the DGCL must do ALL of the following:

Any person who sells Shares in the Offer will not be entitled to exercise appraisal rights with respect thereto but rather will receive the Offer Price, subject to the terms and conditions of the Merger Agreement, as well as the Offer to Purchase and related Letter of Transmittal, as applicable.

The preservation and exercise of appraisal rights require strict and timely adherence to the applicable provisions of Delaware law which will be set forth in detail in the Schedule 14D-9.

The information provided above is for informational purposes only with respect to your alternatives if the Merger is consummated. Any person who desires to exercise his, her or its appraisal rights should review carefully Section 262 of the DGCL and is urged to consult his, her or its legal advisor before electing or attempting to exercise such rights. The foregoing summary does not constitute any legal or other advice nor does it constitute a recommendation that IGM stockholders or beneficial owners of Shares exercise appraisal rights under Section 262 of the DGCL.

If you tender your Shares into the Offer, you will not be entitled to exercise appraisal rights with respect to your Shares but, instead, subject to the conditions to the Offer, you will receive the Offer Price for your Shares.

“Going Private” Transactions. Rule 13e-3 under the Exchange Act is applicable to certain “going private” transactions and may under certain circumstances be applicable to the Merger. However, Rule 13e-3 will be

inapplicable if: (i) the Shares are deregistered under the Exchange Act prior to the Merger or another business combination; or (ii) the Merger or other business combination is consummated within one year after the purchase of the Shares pursuant to the Offer and the amount paid per Share in the Merger or other business combination is at least equal to the amount paid per Share in the Offer. Neither Parent nor Merger Sub believes that Rule 13e-3 will be applicable to the Merger.

Litigation. There have been no lawsuits filed against IGM, the IGM Board, Parent or Merger Sub in connection with the Offer. Lawsuits may be filed against IGM and the IGM Board, and lawsuits may be filed against Parent and Merger Sub, in connection with the Offer, the Merger and the related disclosures. Absent new or different allegations that are material, Parent and Merger Sub will not, and understand that IGM will not, necessarily announce such filings.

Parent has retained the Depositary and Paying Agent and the Information Agent in connection with the Offer. The Depositary and Paying Agent and the Information Agent will receive customary compensation, reimbursement for reasonable out-of-pocket expenses and indemnification against certain liabilities in connection with the Offer, including certain liabilities under the federal securities laws.

As part of the services included in such retention, the Information Agent may contact holders of Shares by personal interview, mail, electronic mail, telephone, telex, telegraph and other methods of electronic communication and may request brokers, dealers, commercial banks, trust companies and other nominees to forward the Offer materials to beneficial holders of Shares.

Except as set forth above, neither Parent nor Merger Sub will pay any fees or commissions to any broker or dealer or other person for soliciting tenders of Shares pursuant to the Offer. Brokers, dealers, commercial banks and trust companies will upon request be reimbursed by us for customary mailing and handling expenses incurred by them in forwarding the offering material to their customers.

The Offer is being made to all holders of the Shares. We are not aware of any jurisdiction in which the making of the Offer or the acceptance thereof would be prohibited by securities, “blue sky” or other valid laws of such jurisdiction. If we become aware of any U.S. state in which the making of the Offer or the acceptance of Shares pursuant thereto would not be in compliance with an administrative or judicial action taken pursuant to a U.S. state statute, we will make a good faith effort to comply with any such law. If, after such good faith effort, we cannot comply with any such law, the Offer will not be made to (nor will tenders be accepted from or on behalf of) the holders of Shares in such state. In any jurisdictions where applicable laws require the Offer to be made by a licensed broker or dealer, the Offer shall be deemed to be made on behalf of Purchaser by one or more registered brokers or dealers licensed under the laws of such jurisdiction to be designated by Purchaser.

Parent and Merger Sub have filed with the SEC the Schedule TO (including exhibits) in accordance with the Exchange Act, furnishing certain additional information with respect to the Offer, and may file amendments thereto. The Schedule TO and any amendments thereto, including exhibits, may be examined and copies may be obtained from the SEC in the manner set forth in “The Tender Offer—Section 5. Certain Information Concerning IGM.”

The Offer does not constitute a solicitation of proxies for any meeting of IGM stockholders. Any solicitation of proxies which Purchaser or any of its affiliates might seek would be made only pursuant to separate proxy materials complying with the requirements of Section 14(a) of the Exchange Act.

No person has been authorized to give any information or make any representation on behalf of Parent or Merger Sub not contained in this Offer to Purchase or in the Letter of Transmittal and, if given or made, such information or representation must not be relied upon as having been authorized. No broker, dealer, bank, trust company, fiduciary or other person shall be deemed to be an agent of Parent, Merger Sub, the Depositary and Paying Agent or the Information Agent for the purpose of the Offer. Neither delivery of this Offer to Purchase nor any purchase pursuant to the Offer will, under any circumstances, create any implication that there has been no change in the affairs of Parent, Merger Sub, IGM or any of their respective subsidiaries since the date as of which information is furnished or the date of this Offer to Purchase.

Concentra Biosciences, LLC’s principal business is currently to consummate the Offer and effect the Merger pursuant to the Merger Agreement, and to perform its obligations under the CVR Agreement following the Merger when IGM is a wholly owned subsidiary of Parent as the surviving entity from the Merger. The following table sets forth information about the executive officers of Concentra Biosciences, LLC as of July 15, 2025.

The common business address and telephone number for the sole director and executive officers of Purchaser is as follows: c/o Concentra Biosciences, LLC, Inc., 4747 Executive Drive, Suite 210, San Diego, California 92121, Tel. (858) 281-5372.

Concentra Merger Sub V, Inc. was incorporated for the purpose of consummating and effecting the Merger pursuant to the Merger Agreement. The following table sets forth information about the sole director and executive officers of Concentra Merger Sub V, Inc. as of July 15, 2025.

The common business address and telephone number for all the directors and executive officers of Merger Sub is as follows: c/o Concentra Merger Sub V, 4747 Executive Drive, Suite 210, San Diego, California 92121, Tel: (858) 281-5372.

Tang Capital Management, LLC is the sole manager of Parent and general partner of TCP. The following table sets forth information about the executive officers of Tang Capital Management, LLC as of July 15, 2025.

The common business address and telephone number for TCM, Mr. Tang, Mr. Hearne and Mr. Cole is as follows: c/o Tang Capital Management, LLC, 4747 Executive Drive, Suite 210, San Diego, California 92121, Tel. (858) 200-3830.

Tang Capital Partners, LP’s principal business is a life sciences-focused investment company. Tang Capital Management, LLC is the general partner of Tang Capital Partners, LP. The business address and telephone number for TCP is as follows: c/o Tang Capital Management, LLC, 4747 Executive Drive, Suite 210, San Diego, California 92121, Tel. (858) 200-3830.

The Letter of Transmittal, certificates (if any) for Shares and any other required documents should be sent by each stockholder of IGM to the Depositary and Paying Agent as follows:

Mail or deliver the Letter of Transmittal, together with the certificate(s) (if any) representing your shares, to:

Questions or requests for assistance or additional copies of this Offer to Purchase, the Letter of Transmittal, and the Schedule TO may be directed to the Information Agent at its location and telephone numbers set forth below. Stockholders may also contact their broker, dealer, commercial bank or trust company for assistance concerning the Offer.

You may call MacKenzie Partners, Inc., the Information Agent for the Offer, toll-free at (800) 322-2885 or email them at tenderoffer@mackenziepartners.com.