Sec Form 4 Filing - Milla Marcos @ Synthorx, Inc. - 2020-01-23

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 Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL OMB Number: 3235-0287 Expires: November 30, 2011 Estimated average burden hours per response... 0.5

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/YY) 01/23/2020 (Street) LA JOLLA, CA92037 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction 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 01/23/2020 U 63,989 D $ 68 0 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction 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 (MM/DD/YY) 7. Title and Amount of Underlying Securities (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 01/23/2020 D 89,237 ( 1 ) 07/31/2027 Common Stock 89,237 $ 67.37 0 D Employee Stock Option (right to buy) $ 0.93 01/23/2020 D 227,806 ( 2 ) 05/16/2028 Common Stock 227,806 $ 67.07 0 D Employee Stock Option (right to buy) $ 11 01/23/2020 D 125,000 ( 3 ) 12/05/2028 Common Stock 125,000 $ 57 0 D

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, CA92037 Chief Scientific Officer Signatures /s/ Christian V. Kuhlen, Attorney-in-Fact 01/24/2020 Signature of Reporting Person Date

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).

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.