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Mersana Therapeutics, Inc.
MRSN
Amendment
Ownership

4.00%

Total Shares

198,720

Issuer CIK

1442836

CUSIP

59045L205

Event Date

Nov 11, 2025

Accepted

Nov 13, 2025, 06:45 PM

SEC EDGARView Original Filing
Reporting Persons (3)
NameType% of ClassAggregateSole VotingShared Voting
BCLS II Investco, LP
Partnership
4.00%198,7200198,720
Bain Capital Life Sciences Fund II, L.P.
Partnership
2.60%131,7760131,776
BCIP Life Sciences Associates, LP
Partnership
0.30%16,049016,049
Disclosure Items (4)

Security Title

Common Stock, par value $0.0001 per share

Issuer Name

Mersana Therapeutics, Inc.

Issuer Address

840 Memorial Drive, Cambridge, MA, 02139

Item 4 of the Initial Statement is hereby amended and supplemented to add the following: As disclosed by the Issuer in its Current Report on Form 8-K, filed with the Securities and Exchange Commission on November 13, 2025, on November 12, 2025 the Issuer entered into an Agreement and Plan of Merger (the "Merger Agreement") by and among the Issuer, Day One Biopharmaceuticals, Inc. ("Parent"), and Emerald Merger Sub, Inc., a direct, wholly owned subsidiary of Parent ("Merger Sub"), pursuant to which Merger Sub will commence a tender offer (the "Offer") to acquire all of the issued and outstanding shares of the Issuer. In connection with the entry into the Merger Agreement, the Reporting Persons entered into a Tender and Support Agreement (the "Support Agreement") with Parent and Merger Sub, dated as of November 12, 2025, pursuant to which the Reporting Persons agreed, among other things, to (i) to tender all of the shares of Common Stock held by the Reporting Persons in the Offer, subject to certain exceptions (including the valid termination of the Merger Agreement), (ii) to, if applicable, vote all of the Reporting Perons's shares of Common Stock in favor of the Merger, and (iii) to certain other restrictions on its ability to take actions with respect to the Company and its Common Stock. The foregoing summary of the Support Agreement does not purport to be complete and is qualified in its entirety by reference to the Support Agreement, which is filed as an exhibit hereto and incorporated by reference herein.

Percentage of Class

Item 5 of the Initial Statement is hereby amended and restated as follows: The information set forth in Items 2 and 3 and on the cover pages of this Schedule 13D is incorporated by reference in its entirety into this Item 5. As of the date hereof, (i) BCLS II holds 131,776 shares of Common Stock, representing approximately 2.6% of the outstanding shares of Common Stock, (ii) BCIPLS holds 16,049 shares of Common Stock, representing approximately 0.3% of the outstanding shares of Common Stock, and (iii) BCLS II Investco holds 198,720 shares of Common Stock, representing approximately 4.0% of the outstanding shares of Common Stock. As a result of the foregoing and the relationships described in Item 2(a) of this Schedule 13D, the Reporting Persons may be deemed to collectively beneficially own an aggregate of 346,545 shares of Common Stock, representing approximately 6.9% of the outstanding shares of Common Stock. The percentage of the outstanding shares of Common Stock held by the Reporting Persons is based on 4,990,692 shares of Common Stock outstanding as of August 8, 2025, as reported by the Issuer in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2025.

Number of Shares

See Item 5(a) hereof.

Transactions

Not applicable.

Shareholders

Except as otherwise described in this Item 5, no one other than the Reporting Persons has the right to receive, or the power to direct the receipt of, dividends from, or the proceeds from the sale of, any of the Common Stock beneficially owned by the Reporting Persons as described in this Item 5.

Date of 5% Ownership

Not applicable.

Exhibit A - Support Agreement (incorporated by reference from Exhibit 2.2 of the Issuer's Current Report on Form 8-K dated November 13, 2025).

Mersana Therapeutics, Inc. — Schedule 13D | 13D Filings