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 March 31, 2019, the total number of shares of common stock available for issuance under the 2013 Plan was approximately 2.4 million.

The Company recognized stock-based compensation expense totaling $32.3 million and $23.0 million for the three months ended March 31, 2019 and 2018, 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,
	2019		2018
Stock options	$	23,183	$	17,295
Restricted stock units		8,881		5,541
Employee stock purchase plan		277		159
	$	32,341	$	22,995

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,
	2019		2018
Research and development	$	15,516	$	11,624
General and administrative		16,825		11,371
	$	32,341	$	22,995

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 March 31, 2019 and none of which vested during the three months ended March 31, 2019. 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 will be first recognized as expense if and when achievement of the regulatory milestone is considered probable. These awards were modified in the second quarter of 2018 as a result of the adoption of a broad-based employee plan. The Company has not recognized any expense related to these awards and may recognize up to $36.2 million in stock-based compensation expense related to these awards upon achievement of the performance condition and subject to the service-based condition.

As of March 31, 2019, the Company had $375.6 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 3.0 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, 2019.

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		1,085		$	135.53
Exercised		(189	)	$	50.31
Canceled, forfeited, or expired		(155	)	$	136.45
Outstanding at March 31, 2019		5,384		$	115.84
Exercisable at March 31, 2019		2,372		$	82.62
Vested and expected to vest at March 31, 2019		5,382		$	115.85

During the three months ended March 31, 2019, 0.2 million shares of common stock were exercised, resulting in total proceeds to the Company of $9.5 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		368		$	135.70
Vested		(131	)	$	131.06
Forfeited		(40	)	$	149.31
Unvested balance at March 31, 2019		1,128		$	152.49

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

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 three months ended March 31, 2019 and 2018, less than 0.1 million shares of common stock were issued under the 2013 ESPP.