Statement of Changes in Beneficial Ownership (4)

FORM 4

[ ] Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES

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3235-0287

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Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

1. Name and Address of Reporting Person * Lowinger Timothy B	2. Issuer Name and Ticker or Trading Symbol Mersana Therapeutics, Inc. [ MRSN ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director                    _____ 10% Owner __X__ Officer (give title below)    _____ Other (specify below) Chief Science, Tech. Officer
(Last)          (First)          (Middle) C/O MERSANA THERAPEUTICS, INC., 840 MEMORIAL DRIVE	3. Date of Earliest Transaction (MM/DD/YYYY) 9/21/2020
(Street) CAMBRIDGE, MA 02139 (City)        (State)        (Zip)	4. If Amendment, Date Original Filed (MM/DD/YYYY)	6. Individual or Joint/Group Filing (Check Applicable Line) _X _ Form filed by One Reporting Person ___ Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security (Instr. 3)	2. Trans. Date	2A. Deemed Execution Date, if any	3. Trans. Code (Instr. 8)		4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	9/21/2020		M(1)		3749	A	$3.51	16160	D
Common Stock	9/21/2020		S(2)		3749	D	$20.6574	12411	D
Common Stock	9/21/2020		M(1)		4454	A	$6.16	16865	D
Common Stock	9/21/2020		S(2)		4454	D	$20.6574	12411	D
Common Stock	9/21/2020		S(2)		12411	D	$20.6574	0	D

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivate Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Trans. Date	3A. Deemed Execution Date, if any	4. Trans. Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Stock Option (right to buy)	$3.51	9/21/2020		M (1)			3749	(3)	1/23/2029	Common Stock	3749	$0	38426	D
Stock Option (right to buy)	$6.16	9/21/2020		M (1)			4454	(4)	1/15/2030	Common Stock	4454	$0	115043	D

Explanation of Responses:
(1)	The stock option exercises reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on August 12, 2020.
(2)	The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on August 12, 2020.
(3)	The options vest in equal quarterly installments over the first four years after the vesting commencement date, January 23, 2019.
(4)	The options vest in equal quarterly installments over the first four years after the vesting commencement date, January 15, 2020.

Reporting Owners
Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Lowinger Timothy B C/O MERSANA THERAPEUTICS, INC. 840 MEMORIAL DRIVE CAMBRIDGE, MA 02139			Chief Science, Tech. Officer

Signatures
/s/ Eva Jack, as Attorney-in-Fact		9/23/2020
**Signature of Reporting Person		Date