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Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

We issue stock-based awards from our equity incentive plans, which are more fully described in Note 9. Stock-based compensation expense was recognized as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016
Cost of goods sold	$	4,629	$	2,333	$	1,620
Research and development		56,193		21,252		13,093
General and administrative		27,279		13,030		11,137
Total stock-based compensation	$	88,101	$	36,615	$	25,850

Black-Scholes Option-Pricing Model Assumptions Used to Calculate Fair Value of Employee Stock Options

The following table lists the Black-Scholes option-pricing model assumptions used to calculate the fair value of employee and director stock options:

	Year Ended December 31,
	2018			2017			2016
Average risk-free interest rate		2.8	%		2.0	%		1.4	%
Dividend yield		0.0	%		0.0	%		0.0	%
Average volatility factor		61.0	%		54.2	%		51.7	%
Average weighted average expected life	5.1 years			5.3 years			5.3 years

Stock Option Activity Under Equity Incentive Plans

The table below presents a summary of stock option activity under our equity incentive plans (in thousands, except for price per share and contractual life information):

	Number of Shares			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Life (in Years)		Aggregate Intrinsic Value(1)
Outstanding at December 31, 2017		18,826		$	20.35
Options granted		1,846			55.35
Options exercised		(4,374	)		13.24
Options forfeited & canceled		(368	)		27.75
Outstanding at December 31, 2018		15,930		$	26.18		4.96	$	214,525

Exercisable at December 31, 2018		9,393			16.16		3.69	$	173,436

(1)	Aggregate intrinsic value represents the difference between the exercise price of the option and the closing market price of our common stock on December 31, 2018.

Restricted Stock Units (RSU) [Member]

Restricted Stock Unit Award Activity

A summary of RSU award activity is as follows (in thousands except for per share amounts):

	Units Issued			Weighted- Average Grant Date Fair Value		Aggregate Intrinsic Value(1)
Balance at December 31, 2017		2,578		$	35.52
Granted		2,125			41.67
Vested and released		(1,255	)		29.44
Forfeited and canceled		(128	)		40.39
Balance at December 31, 2018		3,320		$	41.57	$	109,125

(1)	Aggregate intrinsic value represents the difference between the grant price of the award, which is zero, and the closing market price of our common stock on December 31, 2018.