X0306 4 2020-01-23 1 0001609727 Synthorx, Inc. THOR 0001304113 Powell Andrew Kenneth William C/O SYNTHORX, INC. 11099 N. TORREY PINES ROAD, SUITE 190 LA JOLLA CA 92037 1 0 0 0 Stock Option (right to buy) 14.00 2020-01-23 4 D 0 27015 54.00 D 2028-12-18 Common Stock 27015 0 D Stock Option (right to buy) 12.40 2020-01-23 4 D 0 6389 55.60 D 2029-06-06 Common Stock 6389 0 D This option, which provided for vesting of the shares in equal monthly installments over a three year period such that the option would be fully vested on December 19, 2021, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $1,458,810.00, representing the difference between the exercise price of the option and the market value of the underlying common stock of the Issuer on the effective date of the merger ($68). This option, which provided for vesting of the shares in equal monthly installments over the 12 months following June 7, 2019, such that the option would be fully vested on the date of the Issuer's next annual stockholder meeting, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $355,228.40, representing the difference between the exercise price of the option and the market value of the underlying common stock of the Issuer on the effective date of the merger ($68). /s/ Christian V. Kuhlen, Attorney-in-Fact 2020-01-24