ACHN 2018 Proxy Statement

Votes that are withheld will not be included in the vote tally for the election of directors and will not affect the results of the vote. Proposal 2—Advisory Vote on Executive Compensation To approve Proposal 2, stockholders holding a majority of the shares present or represented and voting on such matter must vote FOR the proposal. 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 2. If you vote to ABSTAIN on Proposal 2, your shares will not be voted in favor of or against the proposal and will also not be counted as votes cast or shares voting on the proposal. As a result, voting to ABSTAIN will have no effect on the voting on the proposal. This vote is advisory only and not binding on the Company. Although this is advisory, we, our Board of Directors, and the Compensation Committee of our Board of Directors value the opinions of our stockholders and expect to take the outcome of this vote into account when considering future compensation arrangements for our executive officers. Proposal 3—Approval of Amendment and Restatement of our 2015 Stock Incentive Plan To approve Proposal 3, stockholders holding a majority of the shares present or represented and voting on such matter must vote FOR the proposal. 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 3. If you vote to ABSTAIN on Proposal 3, your shares will not be voted in favor of or against the proposal and will also not be counted as votes cast or shares voting on the proposal. As a result, voting to ABSTAIN will have no effect on the voting on the proposal. Proposal 4—Ratification of Selection of Independent Registered Public Accounting Firm The Audit Committee of our Board of Directors has selected PricewaterhouseCoopers LLP as our independent registered public accounting firm for the fiscal year 2018. To approve Proposal 4, stockholders holding a majority of the shares present or represented and voting on such matter must vote FOR the proposal. If your shares are held by your broker in “street name,” and you do not vote your shares, your brokerage firm may vote your unvoted shares on Proposal 4. If you vote to ABSTAIN on Proposal 4, your shares will not be voted in favor of or against the proposal and will also not be counted as votes cast or shares voting on the proposal. As a result, voting to ABSTAIN will have no effect on the voting on the proposal. Although stockholder approval of our Audit Committee’s selection of PricewaterhouseCoopers LLP as our independent registered public accounting firm is not required, we believe that it is advisable to give stockholders an opportunity to ratify this selection. If this proposal is not approved at the Annual Meeting, our Audit Committee will reconsider its selection of PricewaterhouseCoopers LLP. Revocability of Proxies You may revoke your proxy at any time before it is exercised. Execution of the proxy will not in any way affect your right to attend the Annual Meeting in person. Revocation may be made prior to the Annual Meeting by either written revocation or through a duly executed proxy bearing a later date sent to Achillion Pharmaceuticals, Inc., Attention: Martha E. Manning, Esq., Secretary, 300 George Street, New Haven, Connecticut 06511, or your proxy may be revoked personally at the Annual Meeting by written notice to the Secretary at the Annual Meeting prior to the voting of the proxy. Any mailed revocation sent to Achillion must include the stockholder’s name and must be received by the day prior to the Annual Meeting to be effective. 2