UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

(Rule 13d-102)

Information Statement Pursuant to Rules 13d-1 and 13d-2

Under the Securities Exchange Act of 1934

(Amendment No. 3)*

Idenix Pharmaceuticals, Inc.

(Name of Issuer)

Common Stock, par value $0.001

(Title of Class of Securities)

45166R204

(CUSIP Number)

December 31, 2012

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this schedule is filed:

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

* 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 purpose 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).

CUSIP No. 45166R204 13G/A Page 2 of 4 Pages

1.	NAME OF REPORTING PERSON Brookside Capital Trading Fund, L.P.* EIN No.: 26-4233731
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a)  ¨        (b)  ¨
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION     Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER     12,664,950 Shares
		6.	SHARED VOTING POWER     0
		7.	SOLE DISPOSITIVE POWER     12,664,950 Shares
		8.	SHARED DISPOSITIVE POWER     0
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON     12,664,950 Shares
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES    ¨
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)     9.46% Shares
12.	TYPE OF REPORTING PERSON     PN

Item 1	(a).	Name of Issuer
		The name of the issuer to which this filing on Schedule 13G/A relates is Idenix Pharmaceuticals, Inc (the “Company”).
Item 1	(b).	Address of Issuer’s Principal Executive Offices
		The principal executive offices of the Company are located at 60 Hampshire Street, Cambridge, MA 02139.
Item 2	(a).	Name of Person Filing
		This Statement is being filed on behalf of Brookside Capital Trading Fund, L.P., a Delaware limited partnership (the “Brookside Fund”). Brookside Capital Investors II, L.P., a Delaware limited partnership (“Brookside Investors II”) is the sole general partner of the Brookside Fund. Brookside Capital Management, LLC, a Delaware limited liability company (“Brookside Management”), is the sole general partner of Brookside Investors II.
Item 2	(b).	Address of Principal Business Office or, if none, Residence
		The principal business address of each of the Brookside Fund, Brookside Investors II, Brookside Management is John Hancock Tower, 200 Clarendon Street, Boston, MA 02116.
Item 2	(c).	Citizenship
		Each of the Brookside Fund, Brookside Investors II and Brookside Management is organized under the laws of the State of Delaware.
Item 2	(d).	Title of Class of Securities
		The class of equity securities of the Company to which this filing on Schedule 13G/A relates is Common Stock, par value $0.001 (“Common Stock”).
Item 2	(e).	CUSIP Number
		The CUSIP number of the Company’s Common Stock is 45166R204.
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
	Not applicable.
	(a)	¨	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o).
	(b)	¨	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 73c).
	(c)	¨	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c).
	(d)	¨	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8).
	(e)	¨	An investment adviser in accordance with §13d-1(b)(1)(ii)(E).
	(f)	¨	An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F).
	(g)	¨	A parent holding company or control person in accordance with §240.13d-1(b)(1)(ii)(G).
	(h)	¨	A savings association as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813).
	(i)	¨	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3).
	(j)	¨	Group, in accordance with §240.13d-1(b)(1)(ii)(J).
		x	If this statement is filed pursuant to §240.13d-1(c), check this box.

Item 4.	Ownership
Item 4	(a).	Amount beneficially owned
		As of the close of business on December 31, 2012, the Brookside Fund beneficially owned 12,664,950 shares of Common Stock.
Item 4	(b).	Percent of Class
		As of the close of business on December 31, 2012, the Brookside Fund beneficially owned 9.46% of the Company’s Common Stock. The aggregate percentage of Common Stock reported owned by the Brookside Fund is based upon 133,882,699 shares of Common Stock outstanding, which is the total number of shares of Common Stock outstanding as of October 24, 2012, based on representations made in the Company’s Form 10-Q filed November 1, 2012 with the Securities and Exchange Commission.
Item 4	(c).	Number of shares as to which such person has:
		(i)	sole power to vote or to direct the vote:
			12,664,950 Shares
		(ii)	shared power to vote or to direct the vote:
			0 Shares
		(iii)	sole power to dispose or to direct the disposition of:
			12,664,950 Shares
		(iv)	shared power to dispose or to direct the disposition of:
			0 Shares
Item 5.	Ownership of Five Percent or Less of a Class
	If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following  ¨.
Item 6.	Ownership of More than Five Percent on Behalf of Another Person
	Not Applicable.
Item 7.	Identification and Classification of the Subsidiary which Acquired the Security Being Reported on by the Parent Holding Company:
	Not Applicable.
Item 8.	Identification and Classification of Members of the Group
	Not Applicable.
Item 9.	Notice of Dissolution of Group
	Not Applicable.
Item 10.	Certification
	By signing below, the undersigned certifies that, to the best of its knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Dated: February 14, 2013

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

BROOKSIDE CAPITAL TRADING FUND, L.P.
By:	/s/ William E. Pappendick IV
	Name: William E. Pappendick
	Title: Managing Director