SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)

12 Months Ended

Dec. 31, 2012

Schedules of Concentration of Risk by Risk Factor

The following table summarizes customers that represent greater than 10% of the Company’s net revenues from research collaborators, for the periods indicated:

	Year Ended December 31,
	2012			2011			2010
Roche/Arrowhead		56	%		68	%		56	%
Takeda		33	%		27	%		22	%

The following table summarizes customers with amounts due that represent greater than 10% of the Company’s billed and unbilled collaboration receivables balance, at the periods indicated:

	At December 31,
	2012			2011
U.S. Government		100	%		*	%
CHDI		—			51	%
GSK		—			20	%
Medtronic		—			13	%

* Represents 10% or less

Fair Value Measured on Recurring Basis

Financial assets and liabilities measured at fair value on a recurring basis are summarized as follows, in thousands:

Description	At December 31, 2012		Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents	$	50,213	$	50,213	$	—	$	—
Marketable securities (fixed income):
Corporate notes		91,523		—		91,523		—
U.S. Government obligations		60,661		—		60,661		—
Commercial paper		22,639		—		22,639		—
Marketable securities (Regulus equity holdings)		38,748		—		38,748		—
Total	$	263,784	$	50,213	$	213,571	$	—

Description	At December 31, 2011		Quoted Prices in Active Markets (Level 1)		Significant Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents	$	67,024	$	67,024	$	—	$	—
Marketable securities (fixed income):
Corporate notes		104,839		—		104,839		—
U.S. Government obligations		73,722		—		73,722		—
Commercial paper		11,395		—		11,395		—
Marketable securities (equity holdings)		625		—		625		—
Total	$	257,605	$	67,024	$	190,581	$	—

Summary of Company's Marketable Securities

The following tables summarize the Company’s marketable securities at December 31, 2012 and 2011, in thousands:

	December 31, 2012
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper (Due within 1 year)	$	22,650	$	1	$	(12	)	$	22,639
Corporate notes (Due within 1 year)		41,249		23		(4	)		41,268
Corporate notes (Due after 1 year through 2 years)		50,322		5		(72	)		50,255
U.S. Government obligations (Due within 1 year)		7,500		—		—			7,500
U.S. Government obligations (Due after 1 year through 2 years)		53,168		2		(9	)		53,161
Total	$	174,889	$	31	$	(97	)	$	174,823

	December 31, 2011
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper (Due within 1 year)	$	11,397	$	—	$	(2	)	$	11,395
Corporate notes (Due within 1 year)		51,273		19		(47	)		51,245
Corporate notes (Due after 1 year through 2 years)		53,592		50		(48	)		53,594
U.S. Government obligations (Due within 1 year)		13,532		2		—			13,534
U.S. Government obligations (Due after 1 year through 2 years)		60,202		7		(21	)		60,188
Equity securities		750		—		(125	)		625
Total	$	190,746	$	78	$	(243	)	$	190,581

Common Shares Excluded From the Calculation of Net Loss Per Common Share

The following table sets forth for the periods presented the potential common shares (prior to consideration of the treasury stock method) excluded from the calculation of net loss per common share because their inclusion would be anti-dilutive, in thousands:

	December 31,
	2012		2011		2010
Options to purchase common stock		8,932		9,779		8,975
Unvested restricted common stock		604		312		113
		9,536		10,091		9,088