Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2012

Fair Value of Financial Assets And Liabilities Measured At Fair Value And Classification By Level Of Input

The following tables summarize the fair value of financial assets and liabilities (investments and convertible preferred stock warrant liabilities) that are measured at fair value, and the classification by level of input within the fair value hierarchy:

	Fair Value Measurements as of December 31, 2012
(in thousands)	Level 1		Level 2		Level 3		Total
Investments:
Money market funds	$	5,923	$	—	$	—	$	5,923
U.S. government-backed securities		—		9,351		—		9,351
Total investments	$	5,923	$	9,351	$	—	$	15,274
Convertible preferred stock warrant liabilites	$	—	$	—	$	157	$	157

	Fair Value Measurements as of December 31, 2011
(in thousands)	Level 1		Level 2		Level 3		Total
Investments:
Money market funds	$	2,973	$	—	$	—	$	2,973
U.S. treasury notes		3,051		—		—		3,051
Corporate securities		—		6,022		—		6,022
U.S. government-backed securities		—		5,436		—		5,436
Total investments	$	6,024	$	11,458	$	—	$	17,482
Convertible preferred stock warrant liabilites	$	—	$	—	$	117	$	117

Inputs Included in Option Pricing Model

The fair value of the convertible preferred stock warrant liabilities through December 31, 2011 was calculated by using an Option Pricing Model to allocate the total enterprise value to the various securities within the Company’s capital structure. The estimated total enterprise value was $106.9 million at December 31, 2011. The model’s inputs reflect assumptions that market participants would use in pricing the instrument in a current period transaction. Inputs to the model included:

Time to liquidity (in years)	2.0
Volatility	53	%
Discounted cash flow rate	14	%
Risk free interest rate	0.25	%
Marketability discount rate	27	%

Fair Value Of Convertible Preferred Stock Warrant Liability

The following table presents changes in financial instruments measured at fair value using Level 3 inputs:

	Convertible Preferred Stock Warrant Liabilites
	(in thousands)
Balance at December 31, 2010	$	69
Unrealized loss included in other income (expense), net		48
Balance at December 31, 2011	$	117
Issuances		79
Unrealized gain included in other income (expense), net		(39	)
Balance at December 31, 2012	$	157

Antidilutive Securities Excluded From Computation Of Earning Per Share

The following shares subject to outstanding potentially dilutive securities have been excluded from the computations of diluted net loss per common share as the effect of including such securities would be antidilutive:

	December 31,
	2012		2011		2010
Convertible preferred stock		12,329,330		10,657,030		10,657,030
Unvested common stock		—		139,033		172,402
Warrants to purchase preferred stock		72,029		38,997		38,997
Options to purchase common stock		1,030,795		1,086,299		962,781
		13,432,154		11,921,359		11,831,210