Vanguard discloses 4.2% Mersana Therapeutics (MRSN) stake in 13G/A

Rhea-AI Filing Summary

The Vanguard Group filed an amended Schedule 13G/A reporting its beneficial ownership in Mersana Therapeutics Inc. common stock as of 12/31/2025. Vanguard reports beneficial ownership of 210,301 shares, representing 4.2% of the outstanding common stock.

Vanguard reports no sole voting or dispositive power, with 26,367 shares subject to shared voting power and 210,301 shares subject to shared dispositive power. The filing states the securities are held in the ordinary course of business and not for the purpose of changing or influencing control of Mersana.

The document notes an internal realignment at Vanguard effective 01/12/2026, after which certain subsidiaries or business divisions that are deemed to have beneficial ownership will report on a disaggregated basis. Vanguard’s clients, including registered investment companies and other managed accounts, have the right to receive dividends and sale proceeds, but no single other person has an interest of more than 5%.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 2)

Mersana Therapeutics Inc

(Name of Issuer)

Common Stock

(Title of Class of Securities)

59045L205

(CUSIP Number)

12/31/2025

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

SCHEDULE 13G

CUSIP No. 59045L205

1

Names of Reporting Persons

The Vanguard Group

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

PENNSYLVANIA

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 26,367.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 210,301.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

210,301.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

4.20 %

12

Type of Reporting Person (See Instructions)

IA

Comment for Type of Reporting Person: On January 12, 2026, The Vanguard Group, Inc. went through an internal realignment. As of that date, The Vanguard Group, Inc. no longer performs portfolio management services or administers proxy voting. In accordance with SEC Release No. 34-39538 (January 12, 1998), The Vanguard Group, Inc. anticipates that certain subsidiaries or business divisions of subsidiaries of The Vanguard Group, Inc., that currently have, or are deemed to have, beneficial ownership with The Vanguard Group, Inc., will report beneficial ownership separately (on a disaggregated basis) from The Vanguard Group, Inc. in reliance on such release. These subsidiaries and/or business divisions pursue the same investment strategies as previously pursued by The Vanguard Group, Inc. prior to the realignment.

SCHEDULE 13G

Item 1.
(a)	Name of issuer: Mersana Therapeutics Inc
(b)	Address of issuer's principal executive offices: 840 Memorial Drive, Cambridge, MA, 02139
Item 2.
(a)	Name of person filing: The Vanguard Group
(b)	Address or principal business office or, if none, residence: 100 Vanguard Blvd., Malvern, PA 19355
(c)	Citizenship: PA
(d)	Title of class of securities: Common Stock
(e)	CUSIP No.: 59045L205
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: 210301
(b)	Percent of class: 4.2 %
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 0
	(ii) Shared power to vote or to direct the vote: 26367
	(iii) Sole power to dispose or to direct the disposition of: 0
	(iv) Shared power to dispose or to direct the disposition of: 210301
Item 5.	Ownership of 5 Percent or Less of a Class.
	Not Applicable
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest relates to more than 5 percent of the class, such person should be identified. A listing of the shareholders of an investment company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment fund is not required. The Vanguard Group, Inc.'s clients, including investment companies registered under the Investment Company Act of 1940 and other managed accounts, have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the securities reported herein. No one other person's interest in the securities reported herein is more than 5%.
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	Not Applicable
Item 8.	Identification and Classification of Members of the Group.
	Not Applicable
Item 9.	Notice of Dissolution of Group.
	Not Applicable

Item 10.

Certifications:

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.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

The Vanguard Group

Signature:	Ashley Grim
Name/Title:	Head of Global Fund Administration
Date:	01/30/2026

FAQ

What ownership stake in Mersana Therapeutics (MRSN) does The Vanguard Group report?

The Vanguard Group reports beneficial ownership of 210,301 Mersana shares, representing 4.2% of the common stock. This stake is reported on a Schedule 13G/A as of December 31, 2025, indicating a sizeable but minority institutional position in the company.

What voting and dispositive power does Vanguard report over Mersana Therapeutics (MRSN) shares?

Vanguard reports no sole voting or dispositive power over Mersana shares. The filing shows shared voting power over 26,367 shares and shared dispositive power over 210,301 shares, reflecting that decisions are made jointly, typically across client accounts and related entities.

Why did The Vanguard Group file an amended Schedule 13G/A for Mersana Therapeutics (MRSN)?

The amendment updates Vanguard’s beneficial ownership position and related disclosures as of December 31, 2025. It also reflects an internal realignment effective January 12, 2026, after which certain Vanguard subsidiaries or business divisions will report beneficial ownership separately on a disaggregated basis.

Does Vanguard intend to influence control of Mersana Therapeutics (MRSN) with this stake?

Vanguard states the Mersana securities were acquired and are held in the ordinary course of business. The filing explicitly notes they are not held for the purpose of changing or influencing control of the issuer, nor in connection with any transaction having that purpose or effect.

Who ultimately benefits from the Mersana Therapeutics (MRSN) shares held by The Vanguard Group?

Vanguard’s clients, including registered investment companies and other managed accounts, benefit from these shares. They have rights to receive dividends and sale proceeds, and the filing specifies that no single other person’s interest in the reported securities exceeds 5% of the class.

How does Vanguard’s internal realignment affect future Mersana Therapeutics (MRSN) ownership reporting?

Following a January 12, 2026 internal realignment, certain Vanguard subsidiaries or business divisions will report beneficial ownership separately. These units pursue the same investment strategies as previously and will rely on SEC Release No. 34-39538 to file on a disaggregated basis going forward.