UNITED STATES
                                 SECURITIES AND EXCHANGE COMMISSION
                                             WASHINGTON, D.C. 20549


    DIVISION OF
CORPORATION FINANCE




                                                            December 23, 2019

  Laura Shawver, Ph.D.
  President and Chief Executive Officer
  Synthorx, Inc.
  11099 N. Torrey Pines Road, Suite 190
  San Diego, California 92037

          Re:     Synthorx, Inc.
                  Schedule 14D-9 filed by Synthorox, Inc. on December 9, 2019
                  File No. 005-90741

  Dear Ms. Shawver:

       We have reviewed the filing referenced above, and we have the following
comment. Our
  comment asks you to provide us with information so we may better understand
your disclosure.

         Please respond to this letter by amending your filing, by providing
the requested
  information or by advising us when you will provide the requested response.
If you do not
  believe our comment applies to your facts and circumstances or do not believe
an amendment is
  appropriate, please tell us why in your response.

         After reviewing any amendment to your filing and the information you
provide in
  response to this comment, we may have additional comments.

  Schedule 14D-9

      1. The Form 8-K filed under cover of Schedule 14D-9 refers to three
exhibits: Exhibit 2.1
         (Agreement and Plan of Merger, dated as of December 7, 2019, by and
among Synthorx,
         Inc., Sanofi and Thunder Acquisition Corp.), Exhibit 99.1 (Joint Press
Release of Sanofi
         and Synthorx, Inc., dated December 8, 2019) and Exhibit 99.2 (Letter
to Synthorx, Inc.
         employees, first made available on December 8, 2019). While those
three exhibits were
         attached to the Form 8-K filed on December 9, 2019, they were not
attached to, or
         incorporated by reference into, the Schedule 14D-9. Although General
Instruction A.2.
         to Form 8-K states that a Form 8-K filing which includes
pre-commencement tender offer
         communications will be deemed filed under certain rules such as Rule
14d-2(b), neither
         the instruction nor the cover page of Form 8-K provides that Form 8-K
may be used to
         perfect compliance with Rule 14d-9(a) with respect to communications
made in advance
         of tender offer commencement. Please explain to us the legal basis
upon which Synthorx
         relied to exclude those exhibits from the Schedule 14D-9, and how the
exclusion thereof
         complies with Rule 14d-9(a)(2). Refer also to General Instruction B.
to Rule 14d-101.
                                         *       *      *


        We remind you that the issuer and its management are responsible for
the accuracy and
adequacy of their disclosures, notwithstanding any review, comments, action or
absence of
action by the staff.

        Please direct any questions to Valian Afshar, Special Counsel, at (202)
551-8729, or me,
at (202) 551-3266.


                                                           Sincerely,

                                                           /s/ Nicholas P.
Panos

                                                           Nicholas P. Panos
                                                           Senior Special
Counsel
                                                           Office of Mergers
and Acquisitions



cc: Barbara Borden, Esq. (via email)
Rama Padmanabhan, Esq. (via email)
Kenneth J. Rollins, Esq. (via email)
Cooley LLP