that are material to Biogen and its subsidiaries, taken as a whole, or to Apellis and its subsidiaries, taken as a whole, that is in effect as of immediately prior to the Expiration Time which
has the effect of prohibiting the consummation of the Offer or making the Merger illegal or otherwise prohibiting consummation of the Offer or the Merger (the “Non-Prohibition Condition”); (iv) the
accuracy of the representations and warranties of Apellis contained in the Merger Agreement as of immediately prior to the Expiration Time , subject to varying materiality thresholds specified therein, including that (A) the representation
regarding the absence of a Company Material Adverse Effect (as defined in the Merger Agreement) since December 31, 2025 must be true and correct in all respects, (B) specified representations regarding capitalization must be true and
correct in all but de minimis respects, (C) specified representations regarding corporate organization, authority, absence of conflict with organizational documents and brokers must be true and correct in all material respects and
(D) all other representations must be true and correct except where the failure to be so would not reasonably be expected to have a Company Material Adverse Effect (collectively, the “Representations Condition”);
(v) Apellis’ compliance with, and performance of, in all material respects all of its covenants and obligations required to be performed or complied with by it under the Merger Agreement at or prior to the Expiration Time (the
“Covenants Condition”); (vi) the receipt by Biogen of a certificate, dated as of the date on which the Acceptance Time (as defined in the Merger Agreement) occurs, signed by an executive officer of Apellis certifying that the
Representations Condition and the Covenants Condition have been satisfied; and (vii) the Merger Agreement not having been validly terminated in accordance with its terms (the “Termination Condition”). The Minimum Condition, the
Regulatory Condition, the Non-Prohibition Condition and the Termination Condition may not be waived by Biogen or Purchaser without the prior written consent of Apellis. Neither the consummation of the Offer
nor the Merger is subject to any financing condition.
The Offer and any withdrawal rights will expire at the end of the
day, Eastern Time, on May 13, 2026 (one minute after 11:59 p.m., Eastern Time, on May 13, 2026) (the “Expiration Time”), unless the Offer is extended pursuant to and in accordance with the Merger Agreement (in which event
the “Expiration Time” will mean the latest time and date at which the Offer, as so extended by Purchaser, will expire).
The board of directors of Apellis unanimously recommends that Apellis stockholders tender their Shares pursuant to the Offer.
The board of directors of Apellis has unanimously (i) determined and declared that it is in the best interests of Apellis and its stockholders that Apellis enter into the Merger Agreement and the CVR Agreement and consummate the Merger and that
the stockholders of Apellis accept the Offer and tender their Shares pursuant to the Offer, in each case on the terms and subject to the conditions set forth in the Merger Agreement, (ii) approved and declared the advisability of the Merger
Agreement, the CVR Agreement, the Offer, the Merger and the other transactions contemplated by the Merger Agreement, (iii) resolved that the Merger will be effected under Section 251(h) of the DGCL, (iv) declared that the terms of the
Offer and the Merger are fair to Apellis and its stockholders and (v) resolved to recommend that Apellis’ stockholders accept the Offer and tender their Shares pursuant to the Offer, in each case, on the terms and subject to the
conditions set forth in the Merger Agreement.
The Merger Agreement contains provisions to govern the circumstances in
which Purchaser is required or permitted to extend the Expiration Time and in which Biogen is required to cause Purchaser to extend the Expiration Time. Specifically, subject to Biogen’s rights to terminate the Merger Agreement in accordance
with its terms, the Merger Agreement provides that Purchaser shall (and Biogen shall cause Purchaser to) extend the scheduled Expiration Time as required by applicable law and extend the Offer for successive extension periods specified by Apellis of
up to ten (10) Business Days (as defined in the Merger Agreement) per extension if, as of the then-scheduled Expiration Time, any Offer Condition (other than solely the Minimum Condition) is not satisfied and has not been waived by Purchaser or
Biogen, to permit such Offer Condition to be satisfied. Purchaser also must extend the Offer for successive extension periods specified by Apellis of up to ten (10) Business Days per extension if, as of the then-scheduled Expiration Time, the
Minimum Condition is not satisfied but all other Offer Conditions (other than those Offer Conditions that by their nature are to be satisfied at the Acceptance Time) have been satisfied or waived, to permit the Minimum Condition to be satisfied,
except that Purchaser will not be required to extend the Offer in the case of the foregoing for more than thirty (30) Business Days in the aggregate. However, Purchaser is not required to, and without Apellis’ written consent, may not,
extend the Offer beyond one minute after 11:59 p.m., Eastern time, on September 30, 2026. If Purchaser extends the Offer, such extension will extend the time that tendering stockholders will have to tender (or withdraw) their Shares.
Biogen and Purchaser expressly reserve the right to increase the Offer Price and the right to waive or make any other changes to
the terms and conditions of the Offer Conditions. However, without the prior written consent of Apellis, Biogen and Purchaser are not permitted to (i) change the form of consideration payable in the Offer, decrease the Offer Price or decrease
the maximum number of Shares sought to be purchased in the Offer, (ii) extend or otherwise change the Expiration Time, except as permitted under the Merger Agreement, (iii) terminate the Offer prior to any scheduled Expiration Time, unless
the Merger Agreement is validly terminated in accordance with its terms, (iv) provide any “subsequent offering period” (or any extension thereof) within the meaning of Rule 14d-11 promulgated
under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), (v) amend, change or waive the Minimum Condition, the Regulatory Condition, the Non-Prohibition Condition or the
Termination Condition, (vi) amend, modify or supplement any Offer Condition or the terms of the Offer in any manner adverse to holders of Shares or that would, individually or in the aggregate, reasonably be expected to have a Parent Material
Adverse Effect (as defined in the Merger Agreement) or (vii) impose any condition to the Offer other than the Offer Conditions.
Any extension, delay, termination or amendment of the Offer will be followed as promptly as practicable by a public announcement
thereof, and such announcement in the case of an extension will be made no later than 9:00 a.m., Eastern Time, on the next business day after the previously scheduled Expiration Time.
Because the Merger will be governed by Section 251(h) of the DGCL, no stockholder vote will be required to consummate the
Merger. Following the consummation of the Offer and subject to the satisfaction or waiver of the remaining conditions set forth in the Merger Agreement, Purchaser, Biogen and Apellis will consummate the Merger as soon as practicable. Purchaser does
not expect there to be a significant period of time between the consummation of the Offer and the consummation of the Merger.
For purposes of the Offer, Purchaser will be deemed to have accepted for payment, and thereby purchased, Shares validly tendered
and not validly withdrawn as, if and when Purchaser gives oral or written notice to the Depositary of Purchaser’s irrevocable acceptance for purchase of such Shares pursuant to the Offer. Upon the terms set forth in the Merger Agreement and
subject to the Offer Conditions, payment for Shares accepted for payment pursuant to the Offer will be made by deposit of the Cash Amount for such Shares with the Depositary, which will act as agent for tendering stockholders for the purpose of
receiving payments from Purchaser and transmitting such payments to tendering stockholders whose Shares have been accepted for payment. For the avoidance of doubt, Biogen is not required to deposit any funds related to any CVR with the Depositary;
neither Purchaser nor Biogen will be required to deposit any funds related to the CVRs with the Rights Agent unless and until such deposit is required pursuant to the terms of the CVR Agreement. If Purchaser extends the Offer, is delayed in its
acceptance for payment of Shares or is unable to accept Shares for payment pursuant to the Offer for any reason, then, without prejudice to Purchaser’s rights under the Offer and the Merger Agreement, the Depositary may, nevertheless, retain
tendered Shares on Purchaser’s behalf, and such Shares may only be withdrawn to the extent that tendering stockholders are entitled to withdrawal rights as described in the Offer to Purchase and as otherwise required by Rule 14e-1(c) under the Exchange Act. Under no circumstances will interest be paid on the purchase price for Shares, regardless of any extension of the Offer or any delay in making payment for Shares. As a result of the
Merger, Apellis will cease to be a publicly traded company and will become a wholly owned subsidiary of Biogen.
At or prior
to the Acceptance Time, Biogen, Apellis and the Rights Agent will execute the CVR Agreement, which will govern the terms of the CVRs. Neither Purchaser nor Biogen will be required to deposit any funds related to the CVRs with the Rights Agent unless
and until such deposit is required pursuant to the terms of the CVR Agreement.
In all cases, payment for Shares validly
tendered, not validly withdrawn and accepted for payment pursuant to the Offer will be made only after timely receipt by the Depositary of (i) the certificates evidencing such Shares (“Share Certificates”), if any, or confirmation
of a book-entry transfer of such Shares (a “Book-Entry Confirmation”) into the Depositary’s account at The Depository Trust Company (the “Book-Entry Transfer Facility”) pursuant to the procedures set forth in the Offer
to Purchase, (ii) the 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 in the Offer to Purchase) in lieu of the
Letter of Transmittal and (iii) any other documents required by the Letter of Transmittal. Accordingly, tendering stockholders may be paid at different times depending upon when the foregoing documents with respect to Shares are actually
received by the Depositary.
Shares tendered pursuant to the Offer may be withdrawn at any time prior to the Expiration Time
and, unless theretofore accepted for payment by Purchaser pursuant to the Offer, may also be withdrawn at any time after June 13, 2026.
For a withdrawal of Shares to be effective, a written notice of withdrawal must be timely received by the Depositary at one of
its addresses set forth on the back cover page of this Offer to Purchase. Any such notice of withdrawal must specify the name of the person who tendered Shares to be withdrawn, the number of Shares to be withdrawn and the name of the registered
holder of such Shares, if different from that of the person who tendered such Shares. If Share Certificates evidencing Shares to be withdrawn have been delivered or otherwise identified to the Depositary, then, prior to the physical release of such
Share Certificates, the serial numbers shown on such Share Certificates must be submitted to the Depositary and the signature(s) on the notice of withdrawal must be guaranteed by an Eligible Institution (as defined in the Offer to Purchase), unless
such Shares have been tendered for the account of an Eligible Institution. If Shares have been tendered pursuant to the procedure for book-entry transfer as set forth in the Offer to Purchase, any notice of withdrawal must also specify the name and
number of the account at the Book-Entry Transfer Facility to be credited with the withdrawn Shares.
Withdrawals of tenders
of Shares may not be rescinded. Any Shares validly withdrawn will thereafter be deemed not to have been validly tendered for purposes of the Offer. However, withdrawn Shares may be re-tendered following one of
the procedures described in the Offer to Purchase at any time prior to the Expiration Time.
All questions as to the form
and validity (including time of receipt) of any notice of withdrawal will be determined by Purchaser, in its sole discretion, whose determination will be final and binding upon the tendering party, subject to the rights of holders of Shares to
challenge such determination with respect to their Shares in a court of competent jurisdiction and any subsequent judgment of any such court. None of Biogen, Purchaser, Apellis, the Depositary, 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 any such notification.
Pursuant to Section 251(h) of the DGCL and due to the obligations of Purchaser, Biogen and Apellis pursuant to the Merger
Agreement to effect the Merger following the consummation of the Offer, Purchaser expects the Merger to occur following the consummation of the Offer without a subsequent offering period. The Merger Agreement does not permit a subsequent offering
period for the Offer without Apellis’ prior written consent.
The information required to be disclosed by paragraph
(d)(1) of Rule 14d-6 of the General Rules and Regulations promulgated under the Exchange Act is contained in the Offer to Purchase and is incorporated herein by reference.
Apellis has provided Purchaser with its stockholder list and security position listings for the purpose of disseminating the
Offer to Purchase, the related Letter of Transmittal and other related materials to holders of Shares. The Offer to Purchase and the related Letter of Transmittal will be mailed to record holders of Shares whose names appear on the stockholder list
of Apellis as of April 1, 2026, and will be furnished, for subsequent transmittal to beneficial owners of Shares, 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.
The exchange of Shares for the Offer Price pursuant to the Offer or the Merger (or for cash upon exercise of appraisal rights)
will be a taxable transaction for U.S. federal income tax purposes. The amount of gain or loss a holder recognizes, and the timing and character of such gain or loss, depend on the U.S. federal income tax treatment of the CVRs and payments (if any)
with respect to the CVRs, with respect to which there is uncertainty. We urge you to consult your own tax advisor as to the particular tax consequences to you of the receipt of cash and CVRs in exchange for Shares pursuant to the Offer or the
Merger, and payments (if any) with respect to the CVRs. See Section 5—“Material U.S. Federal Income Tax Consequences” of the Offer to Purchase for a more detailed discussion of the tax consequences of the Offer and the Merger.
The Offer to Purchase, the Letter of Transmittal and the other related tender offer documents contain important
information. Holders of Shares should carefully read such documents in their entirety before any decision is made with respect to the Offer.
Questions or requests for assistance or additional copies of this Offer to Purchase, the Letter of Transmittal and the notice of
guaranteed delivery may be directed to the Information Agent at its telephone numbers and address set forth below. Stockholders may also contact their broker, dealer, commercial bank, trust company or other nominee for assistance concerning the
Offer.
