X0306 4 2021-12-29 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 Common Stock 2021-12-29 4 P 0 875000 5.75 A 4570667 I See footnotes 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. Dr. Hack is a Managing Director of BCLSI. As a result, Dr. Hack may be deemed to share voting and dispositive power with respect to the securities held by the Bain Capital Life Sciences Entities. Dr. Hack disclaims beneficial ownership of such securities except to the extent of his pecuniary interest therein. /s/ Andrew Hack 2022-01-03