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Stock-Based Compensation (Tables)

6 Months Ended

Jun. 29, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Allocated Employee Stock-Based Compensation Expense

We recorded and allocated employee stock-based compensation expense for our equity incentive plans and our 2000 Employee Stock Purchase Plan (“ESPP”) as follows (in thousands):



	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development	$	2,900	$	1,600	$	5,933	$	3,078
Selling, general and administrative	6,383		3,427		12,655		6,662
Total stock-based compensation	$	9,283	$	5,027	$	18,588	$	9,740

Weighted Average Grant-Date Fair Value of Stock Options and ESPP Purchases

We use the Black-Scholes Merton option pricing model to value our stock options and ESPP purchases. The weighted average grant-date fair value per share of our stock options and ESPP purchases was as follows:



	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Stock options	$	9.82	$	10.07	$	10.37	$	10.01
ESPP	$	6.84	$	5.65	$	7.17	$	4.61

Valuation Assumptions Used to Estimate Grant-Date Fair Value

The grant-date fair value of employee stock option grants and ESPP purchases was estimated using the following assumptions:



	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Stock options:
Risk-free interest rate	2.66	%	1.72	%	2.60	%	1.68	%
Dividend yield	—	%	—	%	—	%	—	%
Volatility	54	%	60	%	54	%	61	%
Expected life	4.5 years		4.3 years		4.4 years		4.2 years
ESPP:
Risk-free interest rate	1.80	%	0.94	%	1.63	%	0.77	%
Dividend yield	—	%	—	%	—	%	—	%
Volatility	52	%	60	%	52	%	64	%
Expected life	6 months		6 months		6 months		6 months

Stock Option Activity

Stock option activity for the six months ended June 30, 2018 was as follows (dollars in thousands, except per share amounts):



	Shares		Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term	Aggregate Intrinsic Value
Options outstanding at December 31, 2017	22,208,446		$	6.83
Granted	818,842		$	22.63
Exercised	(1,141,640	)	$	5.74
Forfeited	(102,177	)	$	15.20
Options outstanding at June 30, 2018	21,783,471		$	7.44	3.8 years	$	312,900
Exercisable at June 30, 2018	16,249,965		$	4.80	3.2 years	$	271,709

RSU Activity

RSU activity for the six months ended June 30, 2018 was as follows (dollars in thousands, except per share amounts):



	Shares		Weighted Average Grant Date Fair Value Per Share		Weighted Average Remaining Contractual Term	Aggregate Intrinsic Value
RSUs outstanding at December 31, 2017	3,762,990		$	17.76
Awarded	300,670		$	23.35
Vested and released	(341,572	)	$	7.75
Forfeited	(120,886	)	$	19.81
RSUs outstanding at June 30, 2018	3,601,202		$	19.11	1.7 years	$	77,498