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SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Name of Issuer)

(Title of Class of Securities)

(CUSIP Number)

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

SCHEDULE 13D/A 0001773187 XXXXXXXX LIVE 8 Common Stock, par value $0.0001 per share 01/06/2026 false 0001442836 59045L205 Mersana Therapeutics, Inc. 840 Memorial Drive Cambridge MA 02139 Bain Capital Life Sciences Inv 617-516-2000 200 Clarendon Street Boston MA 02116 0001773187 N Bain Capital Life Sciences Fund II, L.P. WC N E9 0 0 0 0 0 N 0 PN 0001702921 N BCIP Life Sciences Associates, LP WC N DE 0 0 0 0 0 N 0 PN 0001818893 N BCLS II Investco, LP WC N DE 0 0 0 0 0 N 0 PN Common Stock, par value $0.0001 per share Mersana Therapeutics, Inc. 840 Memorial Drive Cambridge MA 02139 This Amendment No. 8 to Schedule 13D relates to the Common Stock of Mersana Therapeutics, Inc. (the "Issuer") and amends the initial statement on Schedule 13D filed by the Reporting Persons on April 17, 2020, as amended by Amendment No. 1 filed on January 12, 2021, Amendment No. 2 filed on January 3, 2022, Amendment No. 3 filed on February 8, 2022, Amendment No. 4 filed on April 14, 2022, Amendment No. 5 filed on May 11, 2022, Amendment No. 6 filed on November 15, 2024 and Amendment No. 7 filed on November 13, 2025 (the "Initial Statement" and, as further amended by this Amendment No. 8, the "Schedule 13D"). Capitalized terms used but not defined herein shall have the meanings ascribed to them in the Initial Statement. Item 4 of the Initial Statement is hereby amended and supplemented to add the following: As previously disclosed, on November 12, 2025 the Issuer entered into an Agreement and Plan of Merger (the "Merger Agreement") by and among the Issuer, Day One Biopharmaceuticals, Inc. ("Parent"), and Emerald Merger Sub, Inc., a direct, wholly owned subsidiary of Parent ("Purchaser"), pursuant to which, on December 5, 2025, Purchaser commenced a tender offer (the "Offer") to acquire all of the issued and outstanding shares of the Issuer for (i) $25.00 per Share, net to the stockholder in cash without interest and less any applicable tax withholding, plus (ii) one non-tradeable contingent value right per share, which represents the right to receive certain contingent milestone payments of up to an aggregate of $30.25 per share in cash, without interest and less any applicable tax withholding (collectively, the "Offer Price"). On January 6, 2026, Purchaser irrevocably accepted for payment the shares of the Issuer's Common Stock that were validly tendered and not validly withdrawn pursuant to the Offer and, following the consummation of the Offer, pursuant to the terms and conditions of the Merger Agreement, on January 6, 2026, Purchaser was merged with and into the Issuer (the "Merger"), with the Issuer surviving the Merger as a direct wholly-owned subsidiary of Parent. Dr. Hack resigned as a director of the Issuer effective as of the effective time of the Merger (the "Effective Time"). Item 5 of the Initial Statement is hereby amended and restated as follows: The information set forth in Items 2 and 3 and on the cover pages of this Schedule 13D is incorporated by reference in its entirety into this Item 5. Following the Consummation of the Merger on January 6, 2026, the Rerporting Persons ceased to hold any shares of Common Stock. See Item 5(a) hereof. At the Effective Time, pursuant to the terms of the Merger Agreement, all shares of Common Stock held by the Reporting Persons were cancelled and converted into the right to receive the Offer Price, without interest, subject to any applicable withholding tax. Except as otherwise described in this Item 5, no one other than the Reporting Persons has the right to receive, or the power to direct the receipt of, dividends from, or the proceeds from the sale of, any of the Common Stock beneficially owned by the Reporting Persons as described in this Item 5. See Item 5(a) hereof. Bain Capital Life Sciences Fund II, L.P. /s/ Andrew Hack Andrew Hack, Partner of Bain Capital Life Sciences Investors, LLC 01/08/2026 BCIP Life Sciences Associates, LP /s/ Andrew Hack Andrew Hack, Authorized Signatory of Boylston Coinvestors, LLC 01/08/2026 BCLS II Investco, LP /s/ Andrew Hack Andrew Hack, Partner of Bain Capital Life Sciences Investors, LLC 01/08/2026