X0306 4 2020-04-07 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 2020-04-07 4 P 0 2800000 5.59 A 2800000 I See Footnotes Common Stock 2020-04-07 4 P 0 645667 7.74 A 3445667 I See Footnotes Represents shares of common stock held directly by Bain Capital Life Sciences Fund II, L.P. ("BCLS II") and BCIP Life Sciences Associates, LP ("BCIPLS" and, together with BCLS II, the "Bain Life Sciences Entities"). Bain Capital Life Sciences Investors, LLC ("BCLSI") is the ultimate general partner of the BCLS II, and governs the investment strategy and decision-making process with respect to investments held by BCIPLS. Dr. Hack is a Managing Director of BCLSI. By virtue of the relationships described in this footnote, Dr. Hack may be deemed to share voting and dispositive power with respect to the securities held by the Bain Life Sciences Entities. Dr. Hack disclaims beneficial ownership of such securities except to the extent of his pecuniary interest therein. /s/ Andrew A.F. Hack 2020-04-09