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


    DIVISION OF
CORPORATION FINANCE
                                                            December 31, 2019


Via Email

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 Synthorx on December 23, 2019
                File No. 005-90741

Dear Dr. Shawver:

     We have reviewed the above-captioned filing, and have the following
comments. Our
comments may ask you to provide information to us so we may better understand
the 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 comments apply 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 any information you
provide to us in
response to these comments, we may have additional comments.

Schedule 14D-9

Item 3. Past Contacts, Transactions, Negotiations and Agreements, pages 2-13

    1. The beneficial ownership table in footnote (5) on page 4 states that
certain entities "may
       be deemed to have beneficial ownership" of certain shares held by Avalon
Ventures and
       Avalon SPV. Use of the term "deemed" in this context, however, is not
synonymous
       with the term "considered." As such, "deemed" not only has a
distinguishable meaning,
       but also serves a specific legal purpose that ultimately results in the
term intentionally
       only appearing within the regulatory text of Rule 13d-3(b) and Rule
13d-3(d) (which
       subparagraphs, together with subparagraph (a), determine, but do not
define, who is a
       beneficial owner). Accordingly, if such entities beneficially own such
shares, please
       delete the "may be deemed" qualification to remove the implication that
a legal question
       or doubt exists as to whether those entities (and, by virtue of his
shared voting and
       investment power, Jay Lichter) are in fact the beneficial owners of such
shares.
 Laura Shawver, Ph.D.
c/o Synthorx, Inc.
December 31, 2019
Page | 2

   2. We note the following statement: "The Merger Agreement has been filed as
an exhibit to
      this Schedule 14D-9 to provide stockholders with information regarding
its terms and is
      not intended to modify or supplement any factual disclosure about
Synthorx in
      Synthorx's public reports filed with the SEC." Because the Merger
Agreement was filed
      as an exhibit to the Schedule 14D-9, please revise such disclosure to
remove any
      implication that the Merger Agreement and the summary thereof do not
constitute public
      disclosure. Please make conforming changes as necessary throughout this
paragraph.
      See Exchange Act Release No. 34-51283 (March 1, 2005).

Item 4. The Solicitation or Recommendation, pages 13-34

   3. The disclosure on page 13 states that "our Board unanimously recommends
that holders
      of Shares accept the Offer and tender their Shares pursuant to the
Offer." Item 4 of
      Schedule 14D-9 and corresponding Item 1012(a) of Regulation M-A, as well
as Rule
      14e-2(a) of Regulation 14E, require that the "filing person" and "subject
company,"
      respectively, provide the required disclosures. Please revise this
statement (and other
      similar statements throughout this section) to expressly state Synthorx's
position with
      respect to the Offer or alternatively clarify that the Board's
recommendation is being
      made on behalf of Synthorx. See Item 1000(f) of Regulation M-A for a
definition of the
      term "subject company" in the context of Regulation M-A.

   4. The disclosure regarding Company A's unsolicited offer for $70.00 in cash
per share and
      the Board's response thereto appears incomplete. Please supplement such
disclosure to
      indicate whether, as of the date the Schedule 14D-9 was filed, (1) the
Board took any of
      the additional actions permitted to be taken in response to a Superior
Offer by Section
      5.3(c) of the Merger Agreement and (2) Company A took any further action
with respect
      to its unsolicited offer or the Board's response thereto.

   5. Notwithstanding the heading titled "Reasons for Recommendation," we note
the
      disclosure that the Board "considered a number of factors" and that the
"Board
      collectively reached the unanimous conclusion to approve the Merger
Agreement and the
      Transactions in light of these various factors." Item 4 of Schedule 14D-9
and
      corresponding Item 1012(b) of Regulation M-A, however, specifically
require that
      reasons (as distinguished from factors) be disclosed. Please revise
accordingly.

   6. The disclosure on pages 24 and 25 indicates that the Projections contain
non-GAAP
      financial measures, and that reconciliations of such non-GAAP measures to
GAAP
      measures would not be feasible. Please advise us how these disclosures
comply with
      Rule 100 of Regulation G or, alternatively, explain to us why compliance
is not required.

   7. The disclosure on page 27 indicates that Synthorx provided Centerview
with certain
      Internal Data. Please advise us if this Internal Data is different from
the Projections
      disclosed on pages 25 and 26. If such Internal Data is different, please
advise us what
      consideration Synthorx has given to disclosing such Internal Data.
 Laura Shawver, Ph.D.
c/o Synthorx, Inc.
December 31, 2019
Page | 3

   Item 6. Interest in Securities of the Subject Company, page 34

   8. Please advise us how Sythorx concluded that the representation "[n]o
transactions with
      respect to our Shares have been effected [ ] during the [past] 60
days....except for the
      following purchases" is not inconsistent with the disclosure on page 6
regarding the
      December 17 and 18, 2019 grants of equity awards to Drs. Leveque, Milla
and Shawver.

Item 8. Additional Information, pages 35-40

   9. Please refer to the following statement: "This summary does not purport
to be a
      complete statement of, and is qualified in its entirety by reference to,
Section 262 of the
      DGCL [ ]." Although you may include disclaimers concerning the nature of
a summary,
      the summary should not contain omissions of material fact. Please confirm
that no such
      omissions exist, or revise the disclosure as appropriate.

                                        *       *      *

        We remind you that Synthorx 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.
Rama Padmanabhan, Esq.
Kenneth J. Rollins, Esq.