ACHN 2018 Proxy Statement

(4) Consists of (i) 3,234,800 shares of common stock held by OrbiMed Advisors, LLC and (ii) 4,936,173 shares of common stock held by OrbiMed Capital, LLC. OrbiMed Advisors, LLC and OrbiMed Capital, LLC exercise investment and voting power over the shares through a management committee comprised of Carl L. Gordon, Sven H. Borho and Jonathan T. Silverstein, each of whom disclaims beneficial ownership of the shares. OrbiMed Advisors, LLC disclaims beneficial ownership of the shares held indirectly by OrbiMed Capital, LLC, and OrbiMed Capital, LLC disclaims beneficial ownership of the shares held indirectly by OrbiMed Advisors, LLC. This information is from a Schedule 13G/A filed on February 13, 2018. (5) Includes stock options to purchase 230,000 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (6) Includes stock options to purchase 168,750 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (7) Includes stock options to purchase 255,000 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (8) Consists of 106,250 stock options to purchase shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (9) Consists of stock options to purchase 240,000 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018 held by David Scheer and 63,249 shares of common stock held by Scheer Investment Holdings III, LLC. Mr. Scheer, a director of Achillion, is the Managing Member of Scheer Investment Holdings III, LLC. As such, he may be deemed to have sole or shared voting and investment power with respect to the shares held by Scheer Investment Holdings III, LLC. Mr. Scheer disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein. (10) Consists of 240,000 stock options to purchase shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (11) Consists of 80,000 stock options to purchase shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (12) Consists of 3,393,382 shares of common stock held by Domain Partners VIII, L.P., 25,159 shares of common stock held by DP VIII Associates, L.P., 25,000 shares held by Domain Associates, LLC, as well as 31,431 shares of common stock and stock options to purchase 190,000 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018 held by Nicole Vitullo. Nicole Vitullo, a director of Achillion, is a Managing Member of Domain Associates, LLC. Ms. Vitullo disclaims beneficial ownership of such shares except to the extent of her pecuniary interest therein. (13) Includes stock options to purchase 1,869,013 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (14) Includes stock options to purchase 928,562 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (15) Includes stock options to purchase 97,187 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (16) Includes stock options to purchase 923,312 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. (17) Dr. Apelian ceased to be an employee on December 28, 2017. (18) Dr. Barrish ceased to be an employee on July 14, 2017. The information set forth in the table above with respect to Dr. Barrish’s holdings is from a Form 4 filed by Dr. Barrish on December 5, 2016. (19) Includes stock options to purchase 5,328,074 shares of our common stock currently exercisable or exercisable within 60 days of February 28, 2018. None of the securities held by our officers and directors are pledged. 8