Sec Form 4 Filing - Kolchinsky Peter @ 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

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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/YY) 01/23/2020 (Street) BOSTON, MA02116 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 9,724,598 ( 1 ) D $ 68 0 I See Footnote ( 2 ) ( 3 )

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 Stock Option (right to buy) $ 11 01/23/2020 D 30,000 ( 4 ) 12/05/2028 Common Stock 30,000 $ 57 0 D ( 5 ) Stock Option (right to buy) $ 12.4 01/23/2020 D 15,335 ( 6 ) 06/06/2029 Common Stock 15,335 $ 55.6 0 D ( 5 )

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, MA02116 X X Signatures /s/ Peter Kolchinsky, individually 01/24/2020 Signature of Reporting Person Date

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).

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).

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