During the three months ended March 31, 2017, the Company granted 32,500 restricted stock units to certain employees of the Company. The Company did not grant restricted stock units prior to January 1, 2017. These restricted stock units vest ratably over two years, with cliff vesting of 50% at both the one-year and two-year anniversary of the grant date, which was in February 2018 and February 2019, respectively.

During the three months ended March 31, 2018, the Company granted 33,600 performance restricted stock units to certain employees of the Company. The milestones for these grants were not met, and accordingly, these grants were cancelled.

During the three months ended March 31, 2019, the Company granted 354,111 performance restricted stock units to employees of the Company. These performance restricted stock units will vest upon the achievement of a certain clinical and regulatory development milestones related to product candidates and a commercial milestone.

The fair value of restricted stock units that vested during the three months ended March 31, 2019 and 2018 was $2.0 million and $2.6 million, respectively.

	Shares
Outstanding as of December 31, 2018		82,700
Granted		354,111
Vested		(13,250	)
Forfeited		(34,091	)
Outstanding as of March 31, 2019		389,470

On July 2, 2014, the stockholders of the Company approved the 2014 Stock Option and Incentive Plan (the “2014 Stock Option Plan”), which became effective immediately prior to the completion of the Company’s IPO. The 2014 Stock Option Plan provides for the grant of restricted stock awards, restricted stock units, incentive stock options and non-statutory stock options. The 2014 Stock Option Plan replaced the Company’s 2011 Stock Option and Grant Plan (the “2011 Stock Option Plan”). The Company no longer grants stock options or other awards under its 2011 Stock Option Plan, but any options or awards outstanding under the 2011 Stock Option Plan remain outstanding and effective.

The 2014 Stock Option Plan provides for an annual increase, to be added on the first day of each fiscal year, by up to 4% of the Company’s outstanding shares of common stock as of the last day of the prior year. On January 1, 2019, 1,875,530 shares of common stock, representing 4% of the Company’s outstanding shares of common stock as of December 31, 2018, were added to the 2014 Stock Option Plan.

On December 15, 2016, the Board of Directors of the Company (the “Board”) approved the 2016 Inducement Equity Plan (the “2016 Stock Option Plan”). The 2016 Stock Option Plan provides for the grant of equity awards to individuals who have not previously been an employee or a non-employee director of the Company to induce them to accept employment and to provide them with a proprietary interest in the Company. On September 20, 2018, the Board amended the 2016 Stock Option Plan to increase the total number of shares reserved for issuance under such plan by 1,200,000 shares.

As of March 31, 2019, the total number of shares reserved under all equity plans is 11,493,302, and 2,821,785 shares were available for future issuance under such plans.

During the three months ended March 31, 2018, the Company granted 323,753 options to employees to purchase shares of common stock that contain performance-based vesting criteria, primarily related to the achievement of certain clinical and regulatory development milestones related to product candidates and a commercial milestone. Recognition of stock-based compensation expense associated with these performance-based stock options commences when the performance condition is considered probable of achievement, using management’s best estimates, which consider the inherent risk and uncertainty regarding the future outcomes of the milestones.

During the three months ended March 31, 2019, the Company granted no options to employees to purchase shares of common stock that contain performance-based vesting criteria.

During the three months ended March 31, 2019, the achievement of the one commercial milestone that had not been met that is the criteria for vesting of performance-based stock options was considered probable, and therefore $14.2 million of stock-based compensation expense was recognized related to these awards for the three months ended March 31, 2019. Stock options with this milestone were granted during the years ended December 31, 2018 and 2017.

As of March 31, 2018, for grants that were outstanding, the achievement of the milestones that had not been met that are the criteria for vesting of performance-based stock options was considered not probable, and therefore no expense has been recognized related to these awards for the quarter ended March 31, 2018.

Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the three months ended March 31, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
Research and development	$	20,745	$	8,899
General and administrative		23,371		6,918
	$	44,116	$	15,817

Stock-based compensation expense by award type recognized during the three months ended March 31, 2019 and 2018 was as follows:

	Three Months Ended March 31,
	2019		2018
	(in thousands)
Stock options	$	43,564	$	15,381
Restricted stock units		58		168
Employee stock purchase plan		494		268
	$	44,116	$	15,817

The weighted average grant date fair value per share of stock options granted under the Company’s stock option plans during the three months ended March 31, 2019 and 2018 was $98.23 and $125.80, respectively.

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2018		7,530,767		$	93.22		8.03	$	227,447
Granted		1,179,861			155.30
Exercised		(298,102	)		48.32
Forfeited		(130,479	)		120.50
Outstanding as of March 31, 2019		8,282,047		$	103.25		8.10	$	499,599
Exercisable as of March 31, 2019		3,097,197		$	55.80		6.73	$	327,329

At March 31, 2019, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $342.8 million, which is expected to be recognized over the remaining weighted average vesting period of 2.91 years.

The intrinsic value of stock options exercised during the three months ended March 31, 2019 and 2018 was $26.7 million and $47.7 million, respectively.

At March 31, 2019, 745,454 performance-based stock options were both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $49.5 million.