X0306 4 2020-09-14 1 0001442836 Mersana Therapeutics, Inc. MRSN 0001559827 Florence Anthony A. Jr. 5425 WISCONSIN AVENUE, SUITE 800 CHEVY CHASE MD 20815 0 0 1 0 Common Stock 2020-09-14 4 J 0 2000000 0.00 D 5959520 I See Note 2 Common Stock 2020-09-14 4 J 0 20000 0.00 A 20000 I See Note 4 Common Stock 2020-09-14 4 J 0 20000 0.00 D 0 I See Note 4 Common Stock 2020-09-14 4 J 0 10000 0.00 A 10000 I See Note 7 Common Stock 2020-09-14 4 J 0 10000 0.00 D 0 I See Note 7 Common Stock 2020-09-14 4 J 0 39 0.00 A 39 I See Note 10 Common Stock 2020-09-14 4 J 0 5830 0.00 A 5869 I See Note 10 Common Stock 2020-09-14 4 J 0 4391 0.00 A 10260 I See Note 10 Common Stock 2020-09-14 4 J 0 1075 0.00 A 3210 D New Enterprise Associates 14, L.P. ("NEA 14") made a pro rata distribution for no consideration of an aggregate of 2,000,000 shares of Common Stock of the Issuer to its general partner and its limited partners on September 14, 2020. The Reporting Person is a director of NEA 14 GP, LTD ("NEA 14 LTD"), which is the sole general partner of NEA Partners 14, L.P. ("NEA Partners 14"). NEA Partners 14 is the sole general partner of NEA 14, which is the direct beneficial owner of the securities. The Reporting Person disclaims beneficial ownership, within the meaning of Section 16 of the Securities Exchange Act of 1934, as amended, or otherwise of such portion of the NEA 14 shares in which the Reporting Person has no pecuniary interest. NEA Partners 14 received 20,000 shares of Common Stock of the Issuer in the distribution by NEA 14 on September 14, 2020. The Reporting Person is a director of NEA 14 LTD, which is the sole general partner of NEA Partners 14, the direct beneficial owner of the securities. The Reporting Person disclaims beneficial ownership, within the meaning of Section 16 of the Securities Exchange Act of 1934, as amended, or otherwise of such portion of the NEA Partners 14 shares in which the Reporting Person has no pecuniary interest. NEA Partners 14 made a pro rata distribution for no consideration of an aggregate of 20,000 shares of Common Stock of the Issuer to its limited partners on September 14, 2020. NEA 14 Manager Fund, L.P. ("NEA 14 Manager Fund") received 10,000 shares of Common Stock of the Issuer in the distribution by NEA 14 on September 14, 2020. The Reporting Person is a director of NEA 14 LTD, which is the sole general partner of NEA 14 Manager Fund. The Reporting Person disclaims beneficial ownership within the meaning of Section 16 of the Securities Exchange Act of 1934, as amended, or otherwise of such portion of the securities of the Issuer held by NEA 14 Manager Fund in which the Reporting Person has no pecuniary interest. NEA 14 Manager Fund made a pro rata distribution for no consideration of an aggregate of 10,000 shares of Common Stock of the Issuer to its limited partners on September 14, 2020. New Enterprise Associates, LLC ("NEA LLC") received 39 shares of Common Stock of the Issuer in the distribution by NEA 14 on September 14, 2020. The Reporting Person is a member of the Board of Directors of NEA LLC, which is the direct beneficial owner of the shares. The Reporting Person disclaims beneficial ownership within the meaning of Section 16 of the Securities Exchange Act of 1934, as amended, or otherwise of such portion of the shares of the Issuer held by NEA LLC in which the Reporting Person has no pecuniary interest. NEA LLC received 5,830 shares of Common Stock of the Issuer in the distribution by NEA Partners 14 on September 14, 2020. NEA LLC received 4,391 shares of Common Stock of the Issuer in the distribution by NEA 14 Manager Fund on September 14, 2020. The Reporting Person received 1,075 shares of the Common Stock of the Issuer in the distribution of NEA Partners 14 on September 14, 2020. /s/ Sasha Keough, attorney-in-fact 2020-09-16