Under the Securities Exchange Act of 1934
(Amendment No. 10)*

(Name of Issuer)

(Title of Class of Securities)

(CUSIP Number)

(Name, Address and Telephone Number of Person
Authorized to Receive Notices and Communications)

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Sections 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box o.

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purposes of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

(1)  The beneficial ownership percentage is based upon 150,856,104 shares, which the Issuer represented as being outstanding as of the close of business on June 8, 2014 in the Agreement and Plan of Merger among the Issuer, Merck & Co., Inc. and Imperial Blue Corporation, dated as of June 8, 2014 and filed with the Securities and Exchange Commission on June 9, 2014 as Exhibit 2.1 to the Issuer’s Form 8-K.

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(2)  The beneficial ownership percentage is based upon 150,856,104 shares, which the Issuer represented as being outstanding as of the close of business on June 8, 2014 in the Agreement and Plan of Merger among the Issuer, Merck & Co., Inc. and Imperial Blue Corporation, dated as of June 8, 2014 and filed with the Securities and Exchange Commission on June 9, 2014 as Exhibit 2.1 to the Issuer’s Form 8-K.

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Introductory Statement

This Amendment No. 10 (this “Amendment”) amends the Schedule 13D initially filed with the Securities and Exchange Commission on August 6, 2004, as previously amended on September 7, 2005, November 2, 2005, May 20, 2009, June 18, 2010, April 20, 2011, December 15, 2011,  August 10, 2012, February 14, 2014 and March 28, 2014 (the “Original Schedule 13D”) relating to shares of the Common Stock, $0.001 par value per share (the “Common Stock”), of Idenix Pharmaceuticals, Inc., a Delaware corporation (the “Issuer”).  All information reported in the Original Schedule 13D remains in effect except to the extent that it is amended or superseded by information contained in this Amendment.  Capitalized terms used but not defined in this Amendment shall have the respective meanings set forth with respect thereto in the Original Schedule 13D.

Item 4.                                 Purpose of Transaction

The disclosure in Item 4 of the Original Schedule 13D is hereby amended as follows.

On June 20, 2014, Imperial Blue Corporation, a Delaware corporation and a wholly-owned subsidiary of Merck & Co., Inc., commenced a tender offer to purchase all outstanding shares of Common Stock at a price of $24.50 per share in cash (the “Offer”).  On July 28, 2014, the Reporting Persons tendered all shares of Common Stock held by them into the Offer.

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SIGNATURE

After reasonable inquiry and to the best knowledge and belief of the undersigned, the undersigned hereby certify that the information set forth in this statement is true, complete and correct.

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JOINT FILING AGREEMENT

In accordance  with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, each of the persons named below agrees to the joint filing of a Statement on Schedule 13D (including amendments thereto) with respect to the common stock, par value $0.001 per share, of Idenix Pharmaceuticals,  Inc., a Delaware corporation.

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