ACHN 2018 Proxy Statement

Householding of Proxy Materials The Securities and Exchange Commission has adopted rules that permit companies and intermediaries (e.g., brokers) to satisfy the delivery requirements for proxy statements and annual reports with respect to two or more stockholders sharing the same address by delivering a single proxy statement addressed to those stockholders and enclosing separate proxy cards for each stockholder. This process, which is commonly referred to as “householding,” potentially eliminates some duplicative mailings to stockholders and reduces our mailing costs. For this Annual Meeting, a number of brokers with account holders who are stockholders of Achillion will be “householding” our proxy materials. A single proxy statement will be delivered to multiple stockholders sharing an address unless contrary instructions have been received from the affected stockholders. Once you have received notice from your broker that they will be “householding” communications to your address, “householding” will continue until you are notified otherwise or until you revoke your consent. If, at any time, you no longer wish to participate in “householding” and would prefer to receive a separate proxy statement and annual report, please notify your broker, or direct your request via mail to Achillion Pharmaceuticals, Inc., Attention: Martha E. Manning, Esq., Secretary, 300 George Street, New Haven, Connecticut 06511 or via telephone to (203) 624-7000. Upon written or oral request, we will promptly deliver to you a separate proxy statement and annual report. Stockholders who currently receive multiple copies of the proxy statement at their address and would like to request “householding” of their communications should contact their broker. Stockholder Proposals for the 2019 Annual Meeting Proposals of stockholders intended to be presented at the 2019 Annual Meeting of Stockholders must be received by us at our principal office in New Haven, Connecticut not later than December 21, 2018 for inclusion in the proxy statement in accordance with Rule 14a-8 for that meeting. In addition, our by-laws require that we be given advance notice of stockholder nominations for election to our Board of Directors and of other matters which stockholders wish to present for action at an annual meeting of stockholders, other than matters included in our proxy statement in accordance with Rule 14a-8. The required notice must be in writing and received by our Corporate Secretary, Martha E. Manning, Esq., at our principal offices not later than March 2, 2019 or earlier than January 31, 2019. However, if the date of our 2019 annual meeting of stockholders is before May 11, 2019 or after July 30, 2019, notice must be received not earlier than the 120 th day prior to such annual meeting and not later than the close of business on the later of (1) the 90 th day prior to such annual meeting and (2) the 10 th day following the date on which notice of the date of such annual meeting was mailed or public disclosure of the date of such annual meeting was made, whichever occurs first. Our by-laws also specify requirements relating to the content of the notice which stockholders must provide, including a stockholder nomination for election to the Board of Directors, to be properly presented at the 2019 Annual Meeting of Stockholders. 5