X0407 4 2023-06-08 0 0001442836 Mersana Therapeutics, Inc. MRSN 0001657434 Hack Andrew A. F. C/O BAIN CAPITAL LIFE SCIENCES INVESTORS 200 CLARENDON STREET BOSTON MA 02116 1 0 0 0 0 Common Stock 2023-06-08 4 A 0 14467 0.00 A 14467 D Common Stock 8663673 I See footnotes Stock Option (right to buy) 8.64 2023-06-08 4 A 0 18355 0.00 A 2033-06-07 Common Stock 18355 18355 D Represents a restricted stock unit ("RSU") award. Each RSU represents the contingent right to receive one share of Common Stock of the Issuer. The award will vest in full on the earlier of the first anniversary of the date of grant or the date of the 2024 Annual Meeting of Stockholders of the Issuer, subject to the Reporting Person's continuous service with the Issuer on such vesting date. Represents shares of Common Stock held directly by Bain Capital Life Sciences Fund II, L.P. ("BCLS II"), BCIP Life Sciences Associates, LP ("BCIPLS") and BCLS II Investco, LP ("BCLS II Investco" and, together with BCLS II and BCIPLS, the "Bain Capital Life Sciences Entities"). Bain Capital Life Sciences Investors, LLC ("BCLSI") (i) is the ultimate general partner of each of BCLS II and BCLS II Investco and (ii) governs the investment strategy and decision-making process with respect to investments held by BCIPLS. The Reporting Person is a Partner of BCLSI. As a result, the Reporting Person may be deemed to share voting and dispositive power with respect to the securities held by the Bain Capital Life Sciences Entities. The Reporting Person disclaims beneficial ownership of such securities except to the extent of his pecuniary interest therein. /s/ Andrew Hack 2023-06-12