11. Stock-based compensation

In January 2018 and 2017, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 2.0 million and 1.6 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. As of March 31, 2018, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.0 million.

Stock-based compensation expense

The Company recognized stock-based compensation expense totaling $23.0 million and $11.5 million for the three months ended March 31, 2018 and 2017, respectively. Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	For the
	three months ended March 31,
	2018		2017
Stock options	$	17,295	$	9,342
Restricted stock units		5,541		1,956
Employee stock purchase plan		159		183
	$	22,995	$	11,481

Stock-based compensation expense by classification included within the condensed consolidated statements of operations and comprehensive loss was as follows (in thousands):

	For the
	three months ended March 31,
	2018		2017
Research and development	$	11,624	$	5,654
General and administrative		11,371		5,827
	$	22,995	$	11,481

In February 2018, the Company issued 0.3 million restricted stock units with service and performance conditions to employees, none of which vested during the three months ended March 31, 2018. Vesting of these awards is contingent on the occurrence of a certain regulatory milestone event and fulfillment of any remaining service condition. As a result, the related compensation cost is recognized as expense when achievement of the regulatory milestone is considered probable. The Company did not recognize any expense during the three months ended March 31, 2018 related to this award. Upon achievement of the performance condition, and subject to the service-based condition, the Company may recognize up to $51.8 million in stock-based compensation expense.

As of March 31, 2018, the Company had $295.9 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 3.2 years, exclusive of any potential future stock-based compensation expense that may be recognized on any of the Company’s outstanding performance-based awards for which the performance conditions were deemed not probable of achievement as of March 31, 2018.

Stock option activity

The following table summarizes the stock option activity under the Company’s equity award plans:

	Shares (in thousands)			Weighted-average exercise price per share
Outstanding at December 31, 2017		3,755		$	67.91
Granted		1,009		$	203.72
Exercised		(301	)	$	65.53
Canceled or forfeited		(17	)	$	107.83
Outstanding at March 31, 2018		4,446		$	98.91
Exercisable at March 31, 2018		1,844		$	52.56
Vested and expected to vest at March 31, 2018		4,437		$	98.91

During the three months ended March 31, 2018, 0.3 million shares of common stock were exercised, resulting in total proceeds to the Company of $19.7 million. In accordance with the Company’s equity award plans, the shares were issued from a pool of shares reserved for issuance under the equity award plans.

Restricted stock unit activity

The following table summarizes the restricted stock unit activity under the Company’s equity award plans:

	Shares (in thousands)			Weighted-average grant date fair value
Unvested balance at December 31, 2017		477		$	80.72
Granted		522		$	204.44
Vested		(74	)	$	63.26
Forfeited		(5	)	$	128.61
Unvested balance at March 31, 2018		920		$	151.94

Refer above for discussion of the performance-based restricted stock units granted in February 2018, which are included in the table above.

Employee stock purchase plan

On June 3, 2013, the Company adopted its 2013 Employee Stock Purchase Plan (“2013 ESPP”), which authorized the initial issuance of up to a total of 238,000 shares of the Company’s common stock to participating employees. During the three months ended March 31, 2018 and 2017, 9,055 shares and 11,079 shares of common stock were issued under the 2013 ESPP, respectively.