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


   DIVISION OF
CORPORATION FINANCE            ``
                                                            December 31, 2019

Via Email

Karen Linehan
Executive Vice President Legal Affairs and General Counsel
Sanofi
54, Rue La Bo tie, 75008
Paris, France

        Re:      Synthorx, Inc.
                 Schedule TO-T filed on December 23, 2019
                 Filed by Thunder Acquisition Corp., Aventis Inc. and Sanofi
                 File No. 005-90741

Dear Ms. Linehan:

        We have reviewed the above-captioned filing, and have the following
comments. All
defined terms used herein have the same meaning as in the Offer to Purchase
attached as Exhibit
(a)(1)(A) to the Schedule TO-T. In some of our comments, we may ask 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 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 in
response to these comments, we may have additional comments.

Schedule TO-T
Exhibit (a)(1)(A)   Offer to Purchase

General

     1. Please refer to the following statement in the third paragraph of the
cover page: "Under
        no circumstances will interest be paid on the purchase price for the
Shares...." We note,
        however, that shareholders may be entitled to interest on the purchase
price for Shares
        under Section 262 of the DGCL if such shareholders exercise their
appraisal rights in
        connection with the Merger. As such, please revise this statement to
clarify that no
        interest will be paid on the purchase price for Shares purchased in the
Offer, as
        distinguished from Shares purchased in the Merger. Please also make any
conforming
        changes to similar statements throughout the Offer to Purchase.
 Karen Linehan
c/o Sanofi
December 31, 2019
Page | 2

Summary Term Sheet, pages 1-9

    2. The following statement on page 3 states: "We estimate that we will need
approximately
       $2.5 billion to purchase all of the Shares pursuant to the Offer and to
complete the
       Merger." Please advise us why this $2.5 billion figure is different from
the estimated
       transaction valuation of $2,599,172,092 in the "Calculation of Filing
Fee" table in the
       Schedule TO-T. Alternatively, please revise the Offer to Purchase
(including the other
       references to this $2.5 billion figure) or the Schedule TO-T to ensure
consistency.

Acceptance for Payment and Payment for Shares, pages 13-14

    3. The final sentence of this section indicates that "[i]f any tendered
Shares are not
       accepted for payment [ ] unpurchased shares will be returned...as
promptly as
       practicable following the expiration or termination of the Offer." Under
Rule 14e-1(c),
       you must return the Shares "promptly" rather than "as promptly as
practicable" upon the
       expiration or termination of the Offer. Please revise the cited
statement, and make a
       corresponding change with respect to your obligation to pay for Shares
"as promptly as
       practicable after the Offer Acceptance Time" on page 28 of the Offer to
Purchase.

Procedures for Accepting the Offer and Tendering Shares, pages 14-17

    4. Please refer to the following statement: "All questions as to the
validity, form, eligibility
       (including time of receipt) and acceptance for payment of any tender of
Shares will be
       determined by us, in our sole discretion, which determination shall be
final and binding
       on all parties." Please revise this statement to remove the implication
that shareholders
       may not challenge your determinations in a court of competent
jurisdiction. In addition,
       please make conforming changes to similar statements regarding the
"final and binding"
       nature of your determinations throughout the Offer to Purchase.

    5. Please refer to the following statement: "Subject to applicable law as
applied by a court
       of competent jurisdiction and the terms of the Merger Agreement, our
interpretation of
       the terms and conditions of the Offer [ ] will be final and binding."
Please revise to
       expressly indicate that shareholders may challenge your determinations
in a court of
       competent jurisdiction. At present, the disclosure does not account for
this possibility
       and instead only indicates that a court might have a different opinion
that could
       otherwise supersede your "final and binding" determination.

Certain Information Concerning Parent, Purchaser and Certain Related Persons,
pages 22-23

    6. Certain statements regarding Aventis and Parent are qualified by "the
knowledge of
       Purchaser." Because Aventis and Parent are included as Offerors within
the Schedule
       TO, please revise such statements to remove those knowledge qualifiers,
or advise.
 Karen Linehan
c/o Sanofi
December 31, 2019
Page | 3

    7. Please refer to the underfollowing statement: "As of December 20, 2019,
Parent directly
       beneficially owns no Shares." Please revise to clarify whether Parent
beneficially owns
       any Shares, either directly or indirectly.

    8. Please qualify clause (v) in the last paragraph of this section by
reference to the
       contracts, negotiations or transactions between the offerors and the
Company described
       in the Offer to Purchase (or, alternatively by reference to the relevant
sections of the
       Offer to Purchase in which such contracts, negotiations or transactions
are disclosed).

Source and Amount of Funds, page 23

    9. We note that the Offer and the Merger are expected to be funded by
Parent with cash on
       hand. Notwithstanding this disclosure, please affirmatively state
whether or not any
       alternative financing plan exists. See Item 1007(b) of Regulation M-A.

The Merger Agreement; Other Agreements, pages 27-47

    10. Please refer to the following statement: "The summary description . . .
is not intended to
        modify or supplement . . . any factual information about Parent,
Purchaser or the
        Company or the transactions contemplated in the Merger Agreement
contained in public
        reports filed by Parent or the Company with the SEC." We further note,
however, that
        the Merger Agreement was filed as an exhibit to the Schedule TO. As
such, 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, including with respect to the reference to
representations and warranties in
        the subsequent paragraph. See Exchange Act Release No. 34-51283 (March
1, 2005).

Conditions of the Offer, pages 49-51

    11. All offer conditions, other than those dependent upon the receipt of
government
        regulatory approvals necessary to complete the Offer, must be satisfied
or waived at or
        prior to expiration of the Offer. Without an express reference to a
temporal constraint,
        both clause (i) of the second paragraph and the fifth bullet, however,
indicate that you
        may terminate the Offer and not accept for payment any tendered Shares
at any time if
        the Merger Agreement is terminated in accordance with its terms. Please
remove the
        implication that the Offerors may be relieved of their obligation to
purchase shares
        pursuant to certain, unspecified terms unrelated to the receipt of
government approvals.

    12. Each of the clauses (1) through (4) in the sixth bullet contain the
parenthetical "(subject
        to the applicable materiality standard as set forth in [the applicable
clause])." We note,
        however, that clause (2) requires that the representations and
warranties referred to
        therein be accurate without regard to any materiality standard. Please
advise us where
        we can review the materiality standard to which this parenthetical
refers, and explain to
        us what legal effect this parenthetical has on the condition.
 Karen Linehan
c/o Sanofi
December 31, 2019
Page | 4


    13. Clause (4) in the sixth bullet contains the parenthetical "(except in
the case of the
        standard for what constitutes a defined term hereunder and the use of
such defined term
        herein)." Please advise us what legal effect this parenthetical has on
the condition.

    14. All offer conditions, except those dependent upon the receipt of
government regulatory
        approvals necessary to consummate the Offer, must be satisfied or
waived at or before
        the expiration of the Offer. The Offerors, however, represent that the
offer conditions
        "may be waived by Parent or Purchaser in whole or in part at any time
and from time to
        time [ ] subject [to] applicable law." Notwithstanding the reference to
"applicable law,"
        please revise to remove the implication that conditions may be waived
after expiration.

Appraisal Rights, pages 53-54

    15. Please refer to the following statement: "The foregoing summary of the
appraisal rights
        [ ] does not purport to be a complete statement of the procedures to be
followed by the
        stockholders desiring to exercise any appraisal rights [ ] and is
qualified in its entirety
        by reference to Section 262." Although you may include appropriate
disclaimers
        concerning the nature of a summary, the summary must not contain
material omissions.
        Please confirm that no such omissions exist, or revise the disclosure
as appropriate.

Miscellaneous, pages 54-55

    16. We note the following statement: "The Offer is not being made to, nor
will tenders be
        accepted from [ ], holders of Shares in any jurisdiction in which the
making of the Offer
        or the acceptance thereof would not be in compliance with the
securities, blue sky or
        other laws of such jurisdiction." While offer materials need not be
disseminated into
        jurisdictions where such a distribution would be impermissible, please
remove the
        implication that tendered Shares will not be accepted from all
stockholders. See Rule
        14d-10 and guidance in Section II.G.1 of Exchange Act Release No. 58597
(September
        19, 2008). Please make conforming changes to the Letter of Transmittal.

                                         *      *       *

        We remind you that the Offerors and their management are responsible
for the accuracy
and adequacy of their disclosures, notwithstanding any review, comments, action
or absence of
action by the staff.
 Karen Linehan
c/o Sanofi
December 31, 2019
Page | 5

        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: Michael J. Aiello, Esq.
Amanda Fenster, Esq.