Exhibit 3.1

RESTATED CERTIFICATE OF INCORPORATION

OF

IDENIX PHARMACEUTICALS, INC.

FIRST: The name of the corporation is Idenix Pharmaceuticals, Inc. (hereinafter, the “Corporation”).

SECOND: The address of the corporation’s registered office is Corporation Trust Center, 1209 Orange Street, Wilmington, New Castle County, Delaware 19801, and the name of its registered agent at such address is The Corporation Trust Company.

THIRD: The purpose of the corporation is to engage in any lawful act or activity for which a corporation may be organized under the Delaware General Corporation Law.

FOURTH: The total number of shares of stock which the Corporation shall have authority to issue is 100 shares of common stock, par value $0.01 per share.

FIFTH: The business and affairs of the Corporation shall be managed by or under the direction of the board of directors, and the directors need not be elected by ballot unless required by the bylaws of the Corporation.

SIXTH: In furtherance and not in limitation of the powers conferred by the laws of the State of Delaware, the board of directors is expressly authorized to make, amend and repeal the bylaws.

SEVENTH: A director of the Corporation shall not be personally liable to the Corporation or its stockholders for monetary damages for breach of fiduciary duty as a director, except for liability (a) for any breach of the director’s duty of loyalty to the Corporation or its stockholders, (b) for acts or omissions not in good faith or which involve intentional misconduct or a knowing violation of law, (c) under Section 174 of the Delaware General Corporation Law, or (d) for any transaction from which the director derived an improper personal benefit. If the Delaware General Corporation Law is amended to authorize corporate action further eliminating or limiting the personal liability of directors, then the liability of a director of the Corporation shall be eliminated or limited to the fullest extent permitted by the Delaware General Corporation Law, as so amended. Any repeal or modification of this provision shall not adversely affect any right or protection of a director of the Corporation existing at the time of such repeal or modification.

EIGHTH: The Corporation reserves the right to amend and repeal any provision contained in this Certificate of Incorporation in the manner from time to time prescribed by the laws of the State of Delaware. All rights herein conferred are granted subject to this reservation.