ACHN 2018 Proxy Statement

ACHILLION PHARMACEUTICALS, INC. 300 George Street New Haven, Connecticut 06511 Proxy Statement for the 2018 Annual Meeting of Stockholders To Be Held on May 31, 2018 INFORMATION CONCERNING SOLICITATION AND VOTING General This Proxy is solicited on behalf of the Board of Directors of Achillion Pharmaceuticals, Inc., a Delaware corporation (“we,” “Achillion,” “us,” or the “Company”), and contains information about the Annual Meeting of Stockholders to be held on May 31, 2018, at 9:00 a.m., local time, and any adjournment, continuation or postponement of the meeting, referred to throughout this proxy statement as the Annual Meeting, for the purposes set forth herein and in the accompanying Notice of Annual Meeting of Stockholders or any other purpose that may properly come before the Annual Meeting. The Annual Meeting will be held at the offices of the Company at 300 George Street, New Haven, Connecticut 06511. These proxy solicitation materials are being mailed to all stockholders entitled to vote at the Annual Meeting on or about April 20, 2018. Important Notice Regarding the Availability of Proxy Materials for our Annual Meeting to be held on May 31, 2018 This proxy statement and our 2017 Annual Report on Form 10-K are also available to our stockholders electronically via the Internet at www.achillionproxymaterials.com . Purpose of Annual Meeting As described above, the purpose of the Annual Meeting is to obtain approval for the proposals described herein and such other business as may properly come before the meeting, including any adjournment or postponement thereof. Record Date and Shares Outstanding Only stockholders who owned shares of our common stock at the close of business on April 16, 2018, referred to in this proxy statement as the Record Date, are entitled to notice of, and to vote at, the Annual Meeting. Except as otherwise provided in this proxy statement, the holders of common stock as of the Record Date are entitled to one vote per share on matters presented at the Annual Meeting. As of the Record Date, 138,339,738 shares of our common stock were issued and outstanding. Vote Required Proposal 1—Election of Two Class III Directors The two nominees for director to receive the highest number of votes FOR election will be elected as directors. This is called a plurality. If your shares are held by your broker in “street name,” and you do not vote your shares, your brokerage firm may not vote your unvoted shares on Proposal 1. Abstentions are not counted for purposes of electing directors. You may: • vote FOR all nominees; • WITHHOLD your vote from all nominees; or • vote FOR one or more nominees and WITHHOLD your vote from one or more nominees. 1