THOR Synthorx Inc

CUSIP No. 87167A103 

¹ The reporting person is the beneficial owner of 9,724,598 shares of the Issuer’s Common Stock which constitute approximately 30.37% of the class outstanding. The percentage calculation assumes that there are currently 32,017,228 outstanding shares of Common Stock of the Issuer, based upon the closing of the Issuer’s initial public offering, as reported in the Issuer’s prospectus filed with the Securities and Exchange Commission (“SEC”) on December 7, 2018.

CUSIP No. 87167A103  

² The reporting person is the beneficial owner of 9,724,598 shares of the Issuer’s Common Stock which constitute approximately 30.37% of the class outstanding. The percentage calculation assumes that there are currently 32,017,228 outstanding shares of Common Stock of the Issuer, based upon the closing of the Issuer’s initial public offering, as reported in the Issuer’s prospectus filed with the Securities and Exchange Commission (“SEC”) on December 7, 2018.

CUSIP No. 87167A103  

SCHEDULE 13D

This Schedule 13D (the “Statement”) relates to common stock, par value $0.001 per share (the “Common Stock”), of Synthorx, Inc., a Delaware corporation (the “Issuer”).  The address of the principal executive offices of the Issuer is: 11099 N. Torrey Pines Road, Suite 190, La Jolla, CA 92037.

(a) This Schedule 13D is being filed on behalf of (i) RA Capital Management, LLC (“RA Capital”) and (ii) Peter Kolchinsky (together, the “Reporting Persons”).

The Common Stock reported herein includes 7,968,645 shares held by RA Capital Healthcare Fund, L.P. (the “Fund”) and 1,755,953 shares held in a separately managed account (the “Account”). RA Capital is the general partner of the Fund and serves as investment adviser for the Account. Peter Kolchinsky is the manager of RA Capital. As the investment adviser to the Fund and the Account, RA Capital may be deemed a beneficial owner, for purposes of Section 13(d) of the Securities Exchange Act of 1934 (the “Act”), of any securities of the Issuer owned by the Fund or the Account. As the manager of RA Capital, Dr. Kolchinsky may be deemed a beneficial owner, for purposes of Section 13(d) of the Act, of any securities of the Issuer beneficially owned by RA Capital. RA Capital is a registered investment adviser within the meaning of Rule 13d-1(b)(1)(ii)(E) and Rule 16a-1(a)(v), and Dr. Kolchinsky is a parent or control person of RA Capital within the meaning of Rule 13d-1(b)(1)(ii)(G) and Rule 16a-1(a)(1)(vii). RA Capital and Dr. Kolchinsky disclaim beneficial ownership of the securities reported in this Statement other than for the purpose of determining their obligations under Section 13(d) of the Act. The filing of this Statement shall not be deemed an admission that either RA Capital or Dr. Kolchinsky is the beneficial owner of such securities for any other purpose.

(b) The business address of each of the Reporting Persons is: 20 Park Plaza, Suite 1200, Boston, MA 02116.

(c) The Fund is a private investment vehicle. RA Capital provides investment management services to the Fund and the Account. The principal occupation of Dr. Kolchinsky is investment management.

(d) Neither of the Reporting Persons has, during the last five years, been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

(e) Neither of the Reporting Persons has, during the last five years, been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

(f) See Item 6 of the cover pages.

CUSIP No. 87167A103  

Prior to the Issuer’s initial public offering (the “IPO”), the Reporting Persons caused the Fund and the Account to purchase equity interests in the Issuer’s predecessor in a private placement, and those interests converted into 5,179,143 shares of Common Stock which were automatically converted in connection with the Issuer’s IPO. The Fund and the Account also purchased 4,545,455 shares of Common Stock from the underwriters of the IPO. All purchases were for cash and were funded by working capital of the Fund and the Account.

The Reporting Persons acquired the Common Stock reported herein for investment purposes and not with an intent, purpose or effect of changing control of the Issuer. Although the Reporting Persons currently have no plan or proposal to acquire any additional Common Stock or to dispose of any of the Common Stock reported herein, the Reporting Persons may acquire additional Common Stock from time to time or dispose of Common Stock they beneficially own, consistent with their investment purposes and in amounts to be determined by the Reporting Persons based upon a number of factors, including, without limitation, their ongoing assessment of the Issuer's business prospects, prevailing market conditions, the availability of other investment opportunities, and/or other considerations.

In addition, consistent with their investment purpose, the Reporting Persons may engage in communications with persons associated with the Issuer, including shareholders of the Issuer, officers of the Issuer and/or members of the board of directors of the Issuer, to discuss matters regarding the Issuer, including but not limited to its operations and strategic direction. Dr. Peter Kolchinsky currently serves as a director of the Issuer and therefore will engage in regular discussions with the Issuer’s board of directors and management as part of his duties as a director.

The Reporting Persons have no plans or proposals that relate to, or could result in, any of the matters referred to in paragraphs (a) through (j), inclusive, of the instructions to Item 4 of Schedule 13D.

The Reporting Persons may, however, change their purpose and formulate and implement plans or proposals with respect to the Issuer at any time and from time to time. Any such action may be made by the Reporting Persons alone or in conjunction with other shareholders, potential acquirers, financing sources and/or other third parties and could include one or more purposes, plans or proposals that relate to or would result in actions required to be reported herein in accordance with Item 4 of Schedule 13D.

(a) and (b) See Items 7-11 of the cover pages and Item 2 above.

(c) The following table lists the Reporting Persons’ transactions in Common Stock that were effected during the sixty day period prior to the filing of this Schedule 13D:

CUSIP No. 87167A103  

Shares marked with an * were acquired by the Fund. The remaining shares were acquired for the Account.

(d) Not applicable.

(e) Not applicable.

The Reporting Persons have entered into a standard lock-up agreement with the Issuer and the underwriters of the IPO, prohibiting any sale of certain Common Stock holdings reported herein during the 180 days following the closing of the offering. The Reporting Persons may be released from lock-up prior to the expiration of the lock-up period at the sole discretion of the underwriters.

Exhibit 1 Joint Filing Agreement by and among the Reporting Persons.

CUSIP No. 87167A103  

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: December 14, 2018

RA CAPITAL MANAGEMENT, LLC

By: /s/ Peter Kolchinsky

Peter Kolchinsky

Manager

PETER KOLCHINSKY

/s/ Peter Kolchinsky