Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 2014

Stock option plan activity under the 2000 Plan and the 2009 Plan is summarized as follows:

	Shares Available for Grant
Balance at December 31, 2013		2,004,464
Authorized		3,000,000
Granted		(2,448,500	)
Forfeited		66,052
Expired		6,580
Balance at March 31, 2014		2,628,596

Summary of Stock Option Activity under the Company's Equity-Based Plans

The following table summarizes stock option activity under the Company’s equity-based plans for the three months ended March 31, 2014:

	Number of Shares			Weighted Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2013		6,281,148		$	1.74	8.55	$	53,373
Granted		2,448,500			11.20
Exercised		(590,482	)	0.84
Forfeited		(66,052	)		2.54
Expired		(6,580	)		0.50
Balance at March 31, 2014		8,066,534		$	4.67	8.81	$	134,227

Exercisable at March 31, 2014		1,408,826		$	0.79	7.63	$	28,905

Vested and expected to vest at March 31, 2014		7,322,533		$	3.88	8.76	$	127,667

Summary of Fair Value of Option Grant Estimated using Black-Scholes Option-Pricing Model

The fair value of each option grant is estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions:

	Three Months Ended March 31,
	2014			2013
Expected term (in years)	6.05			6.03
Expected volatility		58.3	%		49	%
Risk-free interest rate		1.80	%		1.13	%
Expected dividend yield		0	%		0	%