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Stock-Based Compensation (Tables)

6 Months Ended

Jun. 28, 2019

Share-based Payment Arrangement [Abstract]

Allocated Employee Stock-Based Compensation Expense

We allocated the 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,
	2019				2018				2019				2018
Research and development	$	5,138			$	2,900			$	9,444			$	5,933
Selling, general and administrative	9,941				6,383				18,164				12,655
Total stock-based compensation	$	15,079			$	9,283			$	27,608			$	18,588

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

The weighted average grant-date fair value per share of stock options and ESPP purchases were as follows:

	Three Months Ended June 30,								Six Months Ended June 30,
	2019				2018				2019				2018
Stock options	$	8.67			$	9.82			$	8.83			$	10.37
ESPP	$	5.00			$	6.84			$	4.80			$	7.17

Valuation Assumptions Used to Estimate Grant-Date Fair Value

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

	Three Months Ended June 30,						Six Months Ended June 30,
	2019			2018			2019			2018
Stock options:
Risk-free interest rate	1.75	%		2.66	%		1.90	%		2.60	%
Dividend yield	—	%		—	%		—	%		—	%
Volatility	48	%		54	%		49	%		54	%
Expected life	4.8 years			4.5 years			4.5 years			4.4 years
ESPP:
Risk-free interest rate	2.30	%		1.80	%		2.41	%		1.63	%
Dividend yield	—	%		—	%		—	%		—	%
Volatility	51	%		52	%		55	%		52	%
Expected life	6 months			6 months			6 months			6 months

Stock Option Activity

Activity for stock options during the six months ended June 30, 2019 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, 2018	22,674,062			$	8.71
Granted	801,166			$	21.10
Exercised	(2,147,426	)		$	4.93
Forfeited	(88,105	)		$	15.32
Expired	(29,278	)		$	22.89
Options outstanding at June 30, 2019	21,210,419			$	9.52			3.5 years		$	258,157
Exercisable at June 30, 2019	15,670,941			$	6.36			2.8 years		$	237,737

RSU Activity

Activity for RSUs during the six months ended June 30, 2019 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, 2018	4,857,334			$	18.42
Awarded	272,660			$	21.72
Vested and released	(366,489	)		$	10.68
Forfeited	(180,351	)		$	18.03
RSUs outstanding at June 30, 2019	4,583,154			$	19.26			1.7 years		$	97,942

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