Sec Form 3 Filing - ORBIMED ADVISORS LLC @ Synthorx, Inc. - 2018-12-06

FORM 3 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 ORBIMED ADVISORS LLC 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) 601 LEXINGTON AVENUE, 54TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 12/06/2018 (Street) NEW YORK, NY10022 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ 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

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 Series C Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 3,952,719 ( 2 ) I See Footnote ( 3 ) ( 4 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other ORBIMED ADVISORS LLC 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK, NY10022 X X OrbiMed Capital GP VII LLC 601 LEXINGTON AVENUE, 54TH FLOOR NEW YORK, NY10022 X Signatures /s/ Douglas Coon, Chief Compliance Officer 12/06/2018 Signature of Reporting Person Date /s/ Douglas Coon, Chief Compliance Officer 12/06/2018 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The shares of Series C Preferred Stock have no expiration date and are convertible at any time at the election of the holder into shares of Common Stock without payment of further consideration. Each share of Series C Preferred Stock will automatically convert on a 1.60224-to-1 basis into shares of Common Stock upon the closing of the Issuer's initial public offering. ( 2 )Share numbers give effect to the 1.60224-to-1 reverse stock split of the Issuer's Common Stock effected on November 26, 2018, which will be effective for the Preferred Stock upon its conversion to Common Stock upon the closing of the Issuer's initial public offering. ( 3 )These securities are held of record by OrbiMed Private Investments VII, LP ("OPI VII"). OrbiMed Capital GP VII LLC ("GP VII") is the sole general partner of OPI VII, and OrbiMed Advisors LLC ("OrbiMed Advisors") is the managing member of GP VII. By virtue of such relationships, GP VII and OrbiMed Advisors may be deemed to have voting and investment power with respect to the shares held by OPI VII and as a result may be deemed to have beneficial ownership of such shares. OrbiMed Advisors exercises investment and voting power through a management committee comprised of Carl L. Gordon, Sven H. Borho and Jonathan T. Silverstein. ( 4 )This report on Form 3 is jointly filed by GP VII and OrbiMed Advisors. Each of the Reporting Persons disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange At of 1934, as amended (the "Exchange Act"), except to the extent of its pecuniary interest therein, if any. This report shall not be deemed an admission that any of the Reporting Persons is a beneficial owner of such securities for the purpose of Section 16 of the Exchange Act, or for any other purpose. The Reporting Persons have designated a representative, currently Peter A. Thompson ("Thompson"), an employee of OrbiMed Advisors, to serve on the Issuer's board of directors. This report shall not be deemed an admission that any of the Reporting Persons or Thompson is a beneficial owner of such securities for the purpose of Section 16 of the Exchange Act, or for any other purpose.

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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