The Company's complete summary of significant accounting policies can be found in "Item 8. Financial Statements and Supplementary Data - Note 3. Basis of Presentation and Significant Accounting Policies" in the audited annual consolidated financial statements included in the 2025 Annual Report. The significant accounting policies summarized and included in the 2025 Annual Report have not materially changed, except as set forth below.

The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to U.S. GAAP as found in the ASC and Accounting Standards Updates ("ASUs") of the Financial Accounting Standards Board ("FASB").

The consolidated financial statements include the financial statements of the Company and its wholly owned subsidiaries. All significant intercompany accounts and transactions have been eliminated upon consolidation. Unless the context otherwise indicates, reference in these notes to the "Company" refer to SELLAS Life Sciences Group, Inc., and its wholly owned subsidiaries, SELLAS Life Sciences Group, Ltd., a privately held Bermuda exempted company, SLSG Limited, LLC, and Sellas Life Sciences Limited. The functional currency of the Company's non-U.S. operations is the U.S. dollar.

These consolidated financial statements and accompanying notes should be read in conjunction with the Company's annual consolidated financial statements and the notes thereto included in the 2025 Annual Report. The accompanying consolidated financial statements as of March 31, 2026 and for the three months ended March 31, 2026 and 2025, are unaudited, but include all adjustments, consisting of normal recurring entries, that management believes to be necessary for a fair presentation of the periods presented. Interim results are not necessarily indicative of results for a full year. Balance sheet amounts as of December 31, 2025 have been derived from the audited financial statements as of that date.

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company's chief operating decision maker ("CODM") is the President & Chief Executive Officer.

The Company views its operations and manages its business as one operating segment, which includes all activities related to the development of novel therapeutics for a broad range of cancer indications. The determination of a single reportable segment is consistent with the consolidated financial information provided to the

CODM. The CODM does not evaluate discrete financial information for each of the Company's clinical product candidates, and views and manages the Company's clinical programs as one consolidated segment for which all operations are centralized.

Segment profit or loss is measured as the Company's net loss as reported on the consolidated statement of operations. As the Company does not currently generate revenues, the CODM evaluates Company performance through the achievement of clinical development goals. The CODM also monitors the Company's cash and cash equivalents as reported on the consolidated balance sheet, net cash used in operations as reported on the consolidated statement of cash flows, and segment expense information in order to make operational decisions, allocate resources, and plan for future activities.

Segment expenses consist of the Company's functional expenses, research and development expenses and general and administrative expenses, as reported in the consolidated statement of operations. Other segment items included in the measure of segment net loss include non-operating income, which primarily relates to interest income. The measure of total segment assets is reported on the consolidated balance sheet as total assets.

The Company measures certain financial assets and liabilities at fair value on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability. A three-tier fair value hierarchy is established as a basis for considering such assumptions and for inputs used in the valuation methodologies in measuring fair value:

Level 2—Inputs other than quoted prices included within Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

As of March 31, 2026 and December 31, 2025, the carrying amounts of the Company’s financial instruments, including cash equivalents and accounts payable, approximate fair value due to the short-term nature of those instruments and were categorized as Level 1. The Company did not transfer any financial instruments into or out of Level 3 classification during the three months ended March 31, 2026 or 2025.

Net loss per share is computed by dividing net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding during each period. The weighted average number of shares of common stock outstanding also includes pre-funded warrants and shares held in abeyance because their exercise requires only nominal consideration for the delivery of the shares. Diluted loss per share includes the effect, if any, from the potential exercise or conversion of securities, such as warrants, stock options and unvested restricted stock that would result in the issuance of incremental shares of common stock. In computing the basic and diluted net loss per share, the weighted average number of shares remains the same for both calculations due to the fact that, when a net loss exists, dilutive shares are not included in the calculation as the impact is anti-dilutive.

The following potentially dilutive securities outstanding have been excluded from the computation of diluted weighted average shares outstanding, as they would be anti-dilutive (in thousands):

In November 2024, the FASB issued ASU 2024-03, Income Statement - Reporting Comprehensive Income - Expense Disaggregation Disclosures, which requires disclosure of disaggregated information about certain income statement expense line items in the notes to the financial statements on an interim and annual basis. ASU 2024-03 will be effective for the annual reporting periods in fiscal years beginning after December 15, 2026, with early adoption permitted. The Company is currently evaluating the impact that the adoption of ASU 2024-03 will have on the consolidated financial statements, but does not expect a material impact upon adoption.

In December 2025, the FASB issued ASU 2025-11, Interim Reporting (Topic 270): Narrow-Scope Improvements, which provides clarity about current interim disclosure requirements and introduces a disclosure principle requiring entities to disclose events since the end of the last annual reporting period that have a material impact on the entity. ASU 2025-11 will be effective for interim reporting periods beginning after December 15, 2027. The Company is currently evaluating the impact that the adoption of ASU 2025-11 will have on the consolidated financial statements, but does not expect a material impact upon adoption.