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Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Stock-Based Compensation Expense

The following table presents stock-based compensation expense in the Company’s consolidated statements of operations (in thousands):

		Years ended December 31,
		2017				2016				2015
Cost of product revenue		$	704			$	341			$	213
Research and development			481				537				336
Selling, general and administrative			5,562				3,717				3,049
Total		$	6,747			$	4,595			$	3,598

Estimated Weighted Average Assumptions

The fair value of share-based awards granted during the years ended December 31, 2017, 2016 and 2015 were calculated using the following estimated assumptions:

		2017		2016		2015
Expected term (years)		6.1		6.7 – 7.1		6.6 – 7.2
Volatility		51.48%		50.85 – 51.01%		50.09 – 51.89%
Risk-free interest rate		1.88 – 1.99%		1.51 – 2.37%		1.67 – 2.03%
Expected dividend yield		—		—		—

Summary of Option Activity

Information regarding option activity for the year ended December 31, 2017 under the Plans is summarized below:

		Options Outstanding				Weighted- Average Exercise Price Per Share				Weighted- Average Remaining Contractual Term (in years)				(in thousands) Aggregate Intrinsic Value
Options outstanding at December 31, 2016			882,748			$	16.88
Granted			101,844				33.38
Exercised			(215,495	)			10.92
Forfeited/cancelled			(34,157	)			20.23
Options outstanding at December 31, 2017			734,940			$	20.80				6.46			$	11,558
Options exercisable at December 31, 2017			420,688			$	15.86				5.28			$	8,720
Vested and expected to vest at December 31, 2017(1)			727,278			$	20.71				6.44			$	11,472

(1) Represents the number of vested options as of December 31, 2017 plus the number of unvested options expected to vest as of December 31, 2017 based on the unvested outstanding options at December 31, 2017 adjusted for estimated forfeiture rates of 8% for awards granted to non-executive level employees and 3% for awards granted to executive level employees.

Summary of Restricted Stock Unit Activity

Information regarding restricted stock unit activity for the year ended December 31, 2017 under the Plans is summarized below:

		Options Outstanding				Weighted- Average Remaining Contractual Term (in years)				(in thousands) Aggregate Intrinsic Value
Restricted stock units outstanding at December 31, 2016			353,838
Granted			293,004
Vested			(114,690	)
Forfeited/cancelled			(26,917	)
Restricted stock units outstanding at December 31, 2017			505,235				2.65			$	18,330
Vested and expected to vest at December 31, 2017(1)			466,201				2.40			$	16,914

(1) Represents the number of vested restricted stock units as of December 31, 2017 plus the number of unvested restricted stock units expected to vest as of December 31, 2017 based on the unvested outstanding restricted stock units at December 31, 2017 adjusted for estimated forfeiture rates of 8% for awards granted to non-executive level employees and 3% for awards granted to executive level employees.

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