SCHEDULE 13G/A 0000935836-24-000773 0001587114 XXXXXXXX LIVE 2 Common Stock 03/31/2025 0001442836 Mersana Therapeutics, Inc. 59045L106 840 Memorial Drive Cambridge MA 02139 Rule 13d-1(b) Rule 13d-1(c) EcoR1 Capital, LLC a DE 0.00 0.00 0.00 0.00 0.00 N 0 IA OO Oleg Nodelman a X1 0.00 0.00 0.00 0.00 0.00 N 0 HC IN EcoR1 Capital Fund Qualified, L.P. DE 0.00 0.00 0.00 0.00 0.00 N 0 PN Mersana Therapeutics, Inc. 840 Memorial Drive, Cambridge, MA 02139 EcoR1 Capital, LLC, a Delaware limited liability company ("EcoR1") EcoR1 Capital Fund Qualified, L.P., a Delaware limited partnership ("Qualified Fund") Oleg Nodelman Qualified Fund is filing this statement jointly with the other reporting persons, but not as a member of a group, and it expressly disclaims membership in a group. In addition, the filing of this Schedule 13G on behalf of Qualified Fund should not be construed as an admission that it is, and it disclaims that it is, a beneficial owner, as defined in Rule 13d-3 under the Act, of any securities covered by this Schedule 13G. Each reporting person also disclaims beneficial ownership of the securities reported herein except to the extent of that person's pecuniary interest therein. 357 Tehama Street #3 San Francisco, CA 94103 See Item 4 of the cover sheet for each reporting person. N IA HC EcoR1: 0 Qualified Fund: 0 Oleg Nodelman: 0 EcoR1: 0% Qualified Fund: 0% Oleg Nodelman: 0% EcoR1: 0 Qualified Fund: 0 Oleg Nodelman: 0 EcoR1: 0 Qualified Fund: 0 Oleg Nodelman: 0 EcoR1: 0 Qualified Fund: 0 Oleg Nodelman: 0 EcoR1: 0 Qualified Fund: 0 Oleg Nodelman: 0 N Y Y Y N EcoR1 is the general partner and investment adviser of investment funds, including Qualified Fund. Mr. Nodelman is the control person of EcoR1. Y N By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11. By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11. EXHIBIT 99.1 - AGREEMENT REGARDING JOINT FILING OF STATEMENT ON SCHEDULE 13D OR 13G EcoR1 Capital, LLC /s/ Oleg Nodelman 05/15/2025 Oleg Nodelman /s/ Oleg Nodelman 05/15/2025 EcoR1 Capital Fund Qualified, L.P. /s/ Oleg Nodelman 05/15/2025