X0306 4 2020-01-23 1 0001609727 Synthorx, Inc. THOR 0001551891 Shawver Laura C/O SYNTHORX, INC. 11099 N. TORREY PINES ROAD, SUITE 190 LA JOLLA CA 92037 1 1 0 0 President and CEO Common Stock 2020-01-08 4 G 0 80706 0.00 D 215595 D Common Stock 2020-01-23 4 U 0 220595 68.00 D 0 D Common Stock 2020-01-23 4 U 0 25000 68.00 D 0 I By Laura K. Shawver Trust DTD 3/20/2000 Employee Stock Option (right to buy) 0.93 2020-01-23 4 D 0 293177 67.07 D 2028-05-16 Common Stock 293177 0 D Employee Stock Option (right to buy) 0.63 2020-01-23 4 D 0 172264 67.37 D 2027-12-06 Common Stock 172264 0 D Employee Stock Option (right to buy) 11.00 2020-01-23 4 D 0 385185 57.00 D 2028-12-05 Common Stock 385185 0 D Includes 5,000 shares of Common Stock that were acquired by the Reporting Person on January 22, 2020 pursuant to the Issuer's Employee Stock Purchase Plan. This option, which provided for vesting of 27,695 shares on April 12, 2019, with the remaining 363,944 shares vesting in equal monthly installments thereafter over the next 36 months, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $19,663,381.39, 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 25% of the shares one year after November 27, 2017, with the balance vesting in equal monthly installments thereafter over the next 36 months, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $11,605,425.68, 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 33.333% of the shares on December 18, 2019, with 66.667% of the shares subject to the option vesting upon certification by the Compensation Committee of the Board of Directors of the Issuer that the Issuer has received the first approval for commercial sale by a regulatory authority of any product candidate of the Issuer in any territory in the world, was cancelled in connection with the merger of the Issuer with Sanofi's wholly owned subsidiary in exchange for a cash payment of $21,955,545.00, 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). /s/ Christian V. Kuhlen, Attorney-in-Fact 2020-01-24