THOR Synthorx Inc

FORM 4 [X] 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 * Kolchinsky Peter	2. Issuer Name and Ticker or Trading Symbol Synthorx, Inc. [ THOR ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director                    __X__ 10% Owner _____ Officer (give title below)    _____ Other (specify below)
(Last)          (First)          (Middle) C/O RA CAPITAL MANAGEMENT, L.P., 200 BERKELEY STREET, 18TH FLOOR	3. Date of Earliest Transaction (MM/DD/YYYY) 1/23/2020
(Street) BOSTON, MA 02116 (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	1/23/2020		U		9724598 (1)	D	$68.00	0	I	See Footnote (2)(3)

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)	$11.00	1/23/2020		D			30000	(4)	12/5/2028	Common Stock	30000	$57.00	0	D (5)
Stock Option (right to buy)	$12.40	1/23/2020		D			15335	(6)	6/6/2029	Common Stock	15335	$55.60	0	D (5)

Explanation of Responses:
(1)	These securities include 7,968,645 shares held by RA Capital Healthcare Fund, L.P. (the "Fund") and 1,755,953 shares held by a separately managed account (the "Account").
(2)	RA Capital Management, L.P. (the "Adviser") is the investment manager for the Fund and the Account. The general partner of the Adviser is RA Capital Management GP, LLC (the "Adviser GP"), of which Dr. Peter Kolchinsky and Mr. Rajeev Shah are the managing members.
(3)	The Adviser, the Adviser GP, Dr. Kolchinsky, and Mr. Shah may be deemed indirect beneficial owners of the reported securities for purposes of Section 13(d) of the Securities Exchange Act of 1934 (the "Act"). They disclaim beneficial ownership of any of the reported securities for the purpose of determining whether they are subject to Section 16 of the Act, however, in reliance on Rule 16a-1(a)(1)(v) and (vii). To the extent that they might be deemed subject to Section 16, they disclaim beneficial ownership of securities held by the Fund for purposes of Rule 16a-1(a)(2), except to the extent of their pecuniary interest therein, and disclaim any pecuniary interest in securities held in the Account for purposes of Rule 16a-1(a)(2).
(4)	This option, which provided for vesting of the shares in equal monthly installments over the 12 months following December 6, 2018, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $1,710,000.00, representing the difference between the exercise price of the option and the market value of the underlying common stock of the Issuer on the effective date of the merger ($68).
(5)	Under the Reporting Person's arrangement with the Adviser, the Reporting Person holds the option for the benefit of the Fund and the Account. The Reporting Person is obligated to turn over to the Adviser any net cash or stock received from the option, which will offset advisory fees owed by the Fund and Account to the Adviser. The Reporting Person therefore disclaims beneficial ownership of the option and underlying common stock.
(6)	This option, which provided for vesting of the shares in equal monthly installments over the 12 months following June 7, 2019, such that the option would be fully vested on the date of the Issuer's next annual stockholder meeting, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $852,626.00, representing the difference between the exercise price of the option and the market value of the underlying common stock of the Issuer on the effective date of the merger ($68).

Reporting Owners
Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Kolchinsky Peter C/O RA CAPITAL MANAGEMENT, L.P. 200 BERKELEY STREET, 18TH FLOOR BOSTON, MA 02116	X	X

Signatures
/s/ Peter Kolchinsky, individually		1/24/2020
**Signature of Reporting Person		Date