Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2013

Accounting Policies [Abstract]

Stock-Based Compensation Expense for Employee and Non-Employee Stock Options and Restricted Stock

During the years ended December 31, 2013, 2012, and 2011, the Company recorded stock-based compensation expense for employee and non-employee stock options and restricted stock, which was allocated as follows in the consolidated statements of operations (in thousands):

	Years Ended December 31,
	2013		2012		2011
Research and development expense	$	2,001	$	605	$	253
General and administrative expense		1,029		137		118
	$	3,030	$	742	$	371

Marketable Securities

Marketable securities at December 31, 2013 consist of the following (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Current:
Certificates of deposit	$	6,920	$	—	$	(5	)	$	6,915
U.S. Treasuries		88,287		8		(1	)		88,294
Non-current:
U.S. Treasuries		27,113		12		—			27,125
	$	122,320	$	20	$	(6	)	$	122,334

Marketable securities at December 31, 2012 consist of the following (in thousands):

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Current:
Certificates of deposit	$	7,386	$	—	$	(2	)	$	7,384
U.S. Treasuries		29,294		1		—			29,295
	$	36,680	$	1	$	(2	)	$	36,679

Cash Equivalents and Marketable Securities Measured at Fair Value on a Recurring Basis

The following table summarizes the cash equivalents and marketable securities measured at fair value on a recurring basis as of December 31, 2013 (in thousands):

	Level 1		Level 2		Level 3		Total
Cash equivalents	$	68,792	$	—	$	—	$	68,792
Marketable securities:
Certificates of deposit		—		6,915		—		6,915
U.S. Treasuries		115,419		—		—		115,419
	$	184,211	$	6,915	$	—	$	191,126

The following table summarizes the cash equivalents and marketable securities measured at fair value on a recurring basis as of December 31, 2012 (in thousands):

	Level 1		Level 2		Level 3		Total
Cash equivalents	$	89,062	$	—	$	—	$	89,062
Marketable securities:
Certificates of deposit		—		7,384		—		7,384
U.S. Treasuries		29,295		—		—		29,295
	$	118,357	$	7,384	$	—	$	125,741

Estimated Useful Lives of Property and Equipment

Property and equipment is stated at cost, and depreciated using the straight-line method over the estimated useful lives of the respective assets:

Laboratory equipment	5 years
Computer equipment and software	3 years
Leasehold improvements	Shorter of asset’s useful life or remaining term of lease
Furniture and fixtures	5 years
Office equipment	5 years

Common Stock Excluded from Calculation of Diluted Net Loss Per Share

The following common stock equivalents were excluded from the calculation of diluted net loss per share applicable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Years ended December 31,
	2013		2012		2011
Convertible preferred stock		-		19,731,564		19,731,564
Stock options		3,846,168		3,145,544		2,693,873
Unvested restricted stock		23,295		160,053		291,666
		3,869,463		23,037,161		22,717,103