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 * Milla Marcos	2. Issuer Name and Ticker or Trading Symbol Synthorx, Inc. [ THOR ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director                    _____ 10% Owner __X__ Officer (give title below)    _____ Other (specify below) Chief Scientific Officer
(Last)          (First)          (Middle) C/O SYNTHORX, INC., 11099 N. TORREY PINES ROAD, SUITE 190	3. Date of Earliest Transaction (MM/DD/YYYY) 1/23/2020
(Street) LA JOLLA, CA 92037 (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		63989	D	$68.00	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
Employee Stock Option (right to buy)	$0.63	1/23/2020		D			89237	(1)	7/31/2027	Common Stock	89237	$67.37	0	D
Employee Stock Option (right to buy)	$0.93	1/23/2020		D			227806	(2)	5/16/2028	Common Stock	227806	$67.07	0	D
Employee Stock Option (right to buy)	$11.00	1/23/2020		D			125000	(3)	12/5/2028	Common Stock	125000	$57.00	0	D

Explanation of Responses:
(1)	This option, which provided for vesting of 25% of the shares one year after August 1, 2017, with the balance vesting in equal monthly installments thereafter over the next 36 months, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $6,011,896.69, 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).
(2)	This option, which provided for vesting of 25% of the shares one year after April 12, 2018, with the balance vesting in equal monthly installments thereafter over the next 36 months, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $15,278,948.42, 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).
(3)	This option, which provided for vesting of 33.333% of the shares on December 17, 2019, with 66.667% of the shares subject to the option vesting upon certification by the Compensation Committee of the Board of Directors of the Issuer that the Issuer has received the first approval for commercial sale by a regulatory authority of any product candidate of the Issuer in any territory in the world, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $7,125,000.00, 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).

Reporting Owners
Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Milla Marcos C/O SYNTHORX, INC. 11099 N. TORREY PINES ROAD, SUITE 190 LA JOLLA, CA 92037			Chief Scientific Officer

Signatures
/s/ Christian V. Kuhlen, Attorney-in-Fact		1/24/2020
**Signature of Reporting Person		Date