Synthorx S-8 POSRegistration of securities for employees (post-effective amendment)

As filed with the Securities and Exchange Commission on January 23, 2020

RegistrationNo. 333-228861

RegistrationNo. 333-230215

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

POST-EFFECTIVE AMENDMENT NO. 1

TO:

FORMS-8

REGISTRATION STATEMENT NO. 333-228861

FORMS-8

REGISTRATION STATEMENT NO. 333-230215

UNDER

THE SECURITIES ACT OF 1933

Synthorx, Inc.

(Exact name of registrant as specified in its charter)

2014 Equity Incentive Plan

2018 Equity Incentive Plan

2018 Employee Stock Purchase Plan

(Full titles of the plans)

John Reed

President

Synthorx, Inc.

11099 N. Torrey Pines Road, Suite 190

La Jolla, California 92037

Telephone: (858)750-4700

(Name, address and telephone number of agent for service)

Copies to:

Michael J. Aiello, Esq.

Amanda Fenster, Esq.

Weil, Gotshal & Manges LLP

767 Fifth Avenue

New York, New York 10153

(212)310-8000

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, anon-accelerated filer a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☒

DEREGISTRATION OF SHARES

Synthorx, Inc., a Delaware corporation (the “Registrant”), is filing with the U.S. Securities and Exchange Commission (the “Commission”) these post-effective amendments (the “Post-Effective Amendments”) to deregister all shares of common stock, par value $0.001 per share, of the Registrant (the “Shares”), previously registered under the following Registration Statements on FormS-8 (the “Registration Statements”), together with any and all plan interests and other securities registered thereunder:

On January 23, 2020, pursuant to the terms of the Agreement and Plan of Merger, dated as of December 7, 2019, by and among the Registrant, Sanofi, a Frenchsociété anonyme (“Sanofi”), and Thunder Acquisition Corp., a Delaware corporation and a wholly owned indirect subsidiary of Sanofi (“Merger Sub”), Merger Sub merged with and into the Registrant (the “Merger”), with the Registrant surviving the Merger as a wholly owned indirect subsidiary of Sanofi.

As a result of the Merger, the Registrant has terminated all offerings and sales pursuant to the Registration Statements. In accordance with an undertaking made by the Registrant in the Registration Statements to remove from registration, by means of a post-effective amendment, any of the Shares registered under the Registration Statements that remain unsold at the termination of the offerings, the Registrant hereby removes from registration the Shares registered but remaining unsold under the Registration Statements.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing onForm S-8 and has duly caused these Post-Effective Amendments to the Registration Statements described above to be signed on its behalf by the undersigned, thereunto duly authorized, in the city of La Jolla, State of California, on January 23, 2020.

Pursuant to the requirements of the Securities Act of 1933, as amended, these Post-Effective Amendments to the Registration Statements have been signed below by the following persons in the capacities and on the dates indicated.