In January 2019 and 2018, the number of shares of common stock available for issuance under the 2013 Stock Option and Incentive Plan (“2013 Plan”) was increased by approximately 2.2 million and 2.0 million shares, respectively, as a result of the automatic increase provision of the 2013 Plan. As of June 30, 2019, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.2 million.

The Company recognized stock-based compensation expense totaling $55.1 million and $87.5 million for the three and six months ended June 30, 2019, respectively. The Company recognized stock-based compensation expense totaling $28.1 million and $51.1 million for the three and six months ended June 30, 2018, respectively. Stock-based compensation expense by award type included within the condensed consolidated statements of operations and comprehensive loss was as follows:

	For the				For the
	three months ended June 30,				six months ended June 30,
(in thousands)	2019		2018		2019		2018
Stock options	$	24,800	$	20,932	$	47,983	$	38,227
Restricted stock units		30,012		6,953		38,893		12,494
Employee stock purchase plan		299		171		576		330
	$	55,111	$	28,056	$	87,452	$	51,051

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

	For the				For the
	three months ended June 30,				six months ended June 30,
(in thousands)	2019		2018		2019		2018
Research and development	$	29,694	$	14,196	$	45,210	$	25,820
General and administrative		25,417		13,860		42,242		25,231
	$	55,111	$	28,056	$	87,452	$	51,051

In February 2018, the Company issued restricted stock units with service and performance conditions to employees, approximately 0.2 million of which are outstanding as of June 30, 2019. Vesting of these awards is contingent on the occurrence of a certain regulatory milestone event which was achieved in June 2019 and fulfillment of any remaining service condition. The Company began recognizing expense for these awards in the second quarter of 2019 when achievement of the regulatory milestone was deemed probable. The Company recognized $20.1 million of expense related to these awards in the second quarter of 2019 and will continue to recognize stock-based compensation expense related to these awards through June 2021 when the final tranche of the awards vest.

As of June 30, 2019, the Company had $372.4 million of unrecognized stock-based compensation expense related to unvested stock options, restricted stock units, performance-based restricted stock units, and the employee stock purchase plan, which is expected to be recognized over a weighted-average period of 2.8 years.

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, 2018		4,643		$	108.56
Granted		1,266		$	136.17
Exercised		(282	)	$	47.80
Canceled, forfeited, or expired		(234	)	$	129.85
Outstanding at June 30, 2019		5,393		$	117.30
Exercisable at June 30, 2019		2,520		$	88.94
Vested and expected to vest at June 30, 2019		5,393		$	117.30

During the six months ended June 30, 2019, 0.3 million shares of common stock were exercised, resulting in total proceeds to the Company of $13.5 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.

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, 2018		931		$	155.99
Granted		438		$	136.32
Vested		(197	)	$	142.82
Forfeited		(69	)	$	153.94
Unvested balance at June 30, 2019		1,103		$	150.67

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 0.2 million shares of the Company’s common stock to participating employees. During each of the six months ended June 30, 2019 and 2018, less than 0.1 million shares of common stock were issued under the 2013 ESPP.