Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 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, 2010
(in thousands)	Level 1		Level 2		Level 3		Total
Investments:
Money market funds	$	7,529	$	—	$	—	$	7,529
U.S. treasury notes		1,274		—		—		1,274
Commercial paper				6,749				6,749
Corporate securities		—		7,475		—		7,475
U.S. government-backed securities		—		10,677		—		10,677
Total investments	$	8,803	$	24,901	$	—	$	33,704
Convertible preferred stock warrant liabilities	$	—	$	—	$	69	$	69

	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 liabilities	$	—	$	—	$	117	$	117

	Fair Value Measurements as of September 30, 2012
(in thousands)	Level 1		Level 2		Level 3		Total
	(unaudited)
Investments:
Money market funds	$	5,733	$	—	$	—	$	5,733
U.S. treasury notes		502		—		—		502
Commercial paper		—		4,248				4,248
Corporate securities		—		5,264		—		5,264
U.S. government-backed securities		—		8,307		—		8,307
Total investments	$	6,235	$	17,819	$	—	$	24,054
Convertible preferred stock warrant liabilities	$	—	$	—	$	584	$	584

Inputs Included in Option Pricing Model

The fair value of the convertible preferred stock warrant liabilities as of December 31, 2010 and 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 $99.5 million and $106.9 million at December 31, 2010 and December 31, 2011, respectively. 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:

	December 31,
	2010		2011
Time to liquidity (in years)		0.5		2.0
Volatility		57%		53%
Discounted cash flow rate		14%		14%
Risk free interest rate		0.21%		0.25%
Marketability discount rate		20%		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 Liabilities
(in thousands)
Balance at December 31, 2009	$	71
Unrealized gain included in other income (expense), net		(2	)
Balance at December 31, 2010		69
Unrealized loss included in other income (expense), net		48
Balance at December 31, 2011		117
Issuances (unaudited)		79
Unrealized loss included in other income (expense), net (unaudited)		388
Balance at September 30, 2012 (unaudited)	$	584

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,						September 30,
	2009		2010		2011		2011		2012
							(unaudited)
Convertible Preferred Stock		10,657,030		10,657,030		10,657,030		10,657,030		12,329,330
Unvested Common Stock		237,641		172,402		139,033		156,785		—
Warrants to purchase Convertible Preferred Stock		38,997		38,997		38,997		38,997		55,513
Options to purchase Common Stock		1,040,658		962,781		1,086,299		1,175,855		1,057,468
		11,974,326		11,831,210		11,921,359		12,028,667		13,442,311

Pro Forma Net Loss Per Common Share

	Year Ended December 31, 2011		Nine Months Ended September 30, 2012
			(unaudited)
Weighted average common shares outstanding used to calculate basic and diluted net loss per common share		1,933,672		2,070,013
Pro forma adjustment to reflect the assumed conversion of preferred stock		10,657,030		11,522,793
Weighted average common shares outstanding used to calculate pro forma basic and diluted net loss per common share		12,590,702		13,592,806