CYCN Cyclerion Therapeutics Inc

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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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 * MCGUIRE TERRANCE	2. Issuer Name and Ticker or Trading Symbol Cyclerion Therapeutics, Inc. [ CYCN ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director                    _____ 10% Owner _____ Officer (give title below)    _____ Other (specify below)
(Last)          (First)          (Middle) C/O CYCLERION THERAPEUTICS, INC., 245 FIRST STREET, 18TH FLOOR	3. Date of Earliest Transaction (MM/DD/YYYY) 6/3/2021
(Street) CAMBRIDGE, MA 02142 (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	6/3/2021		P		92831 (1)	A	$3.12	697313	I	By Polaris Partners VIII, L.P. (2)
Common Stock	6/3/2021		P		3322 (1)	A	$3.12	24957	I	By Polaris Entrepreneurs' Fund VIII, L.P. (3)
Common Stock								162	I	By Polaris Venture Management Co. II, L.L.C. (4)
Common Stock								6911	I	By Bartlett Partners, LLC (5)
Common Stock								11047	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

Explanation of Responses:
(1)	On June 3, 2021, Polaris Partners VIII, L.P. ("PP VIII") and Polaris Entrepreneurs' Fund VIII, L.P. ("PEF VIII") agreed to purchase directly from the Issuer such shares of Common Stock of the Issuer in a private placement transaction.
(2)	The reportable securities are owned directly by PP VIII. Polaris Partners GP VIII, L.L.C. ("PPGP VIII") is the general partner of PP VIII. The Reporting Person, a member of the Issuer's Board of Directors, is an interest holder of PPGP VIII. Each of David Barrett, Brian Chee, Amir Nashat and Bryce Youngren are the managing members of PPGP VIII (collectively, the "Managing Members"). Each of the Reporting Person and the Managing Members, in their respective capacities with respect to PPGP VIII, may be deemed to have shared voting and dispositive power over the shares held by PP VIII. Each of PPGP VIII, the Reporting Person and the Managing Members disclaims beneficial ownership of these securities and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interests therein.
(3)	The reportable securities are owned directly by PEF VIII. PPGP VIII is the general partner of PEF VIII. The Reporting Person, a member of the Issuer's Board of Directors, is an interest holder of PPGP VIII. Each of the Reporting Person and the Managing Members, in their respective capacities with respect to PPGP VIII, may be deemed to have shared voting and dispositive power over the shares held by PEF VIII. Each of PPGP VIII, the Reporting Person and the Managing Members disclaims beneficial ownership of these securities and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interests therein.
(4)	The Reporting Person and Jonathan Flint ("Flint") are the managing members of Polaris Venture Management Co. II, L.L.C., the beneficial owner of the securities. Each of the Reporting Person and Flint disclaims beneficial ownership of these securities and this report shall not be deemed an admission that any one of them is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of their respective pecuniary interests therein.
(5)	The Reporting Person is a managing member of Bartlett Partners, LLC, the beneficial owner of the securities. The Reporting Person disclaims beneficial ownership of these securities and this report shall not be deemed an admission that he is the beneficial owner of such securities for purposes of Section 16 or for any other purpose, except to the extent of his pecuniary interest therein.

Reporting Owners
Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
MCGUIRE TERRANCE C/O CYCLERION THERAPEUTICS, INC. 245 FIRST STREET, 18TH FLOOR CAMBRIDGE, MA 02142	X

Signatures
/s/ Lauren Crockett Attorney-in-Fact		6/7/2021
**Signature of Reporting Person		Date