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 vested 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. The fair value of restricted stock units that vested during the three months ended March 31, 2019 was $2.0 million, and since these vested, no restricted stock units have been outstanding. During the year ended December 31, 2018, the Company granted 37,800 performance restricted stock units that will vest upon the achievement of a certain commercial milestone.

During the three months ended March 31, 2020 and 2019, the Company granted 393,811 and 354,111 performance restricted stock units, respectively, to employees of the Company. These performance restricted stock units are related to the achievement of certain clinical and regulatory development milestones related to product candidates and commercial milestones.

Recognition of stock-based compensation expense associated with these performance restricted stock units 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.

No performance restricted stock units vested during the three months ended March 31, 2020 and 2019.

	Shares
Outstanding as of December 31, 2019		333,243
Granted		393,811
Vested		—
Forfeited		(22,423	)
Outstanding as of March 31, 2020		704,631

At March 31, 2020, 704,631 performance restricted stock units were both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $61.8 million.

On July 2, 2014, the stockholders of the Company approved the 2014 Stock Option and Incentive Plan (the “2014 Plan”), which became effective immediately prior to the completion of the Company’s IPO. The 2014 Plan provides for the grant of restricted stock awards, restricted stock units, incentive stock options and non-statutory stock options. The 2014 Plan replaced the Company’s 2011 Stock Option and Grant Plan (the “2011 Plan”). The Company no longer grants stock options or other awards under its 2011 Plan, but any options outstanding under the 2011 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, 2020, 2,075,087 shares of common stock, representing 4% of the Company’s outstanding shares of common stock as of December 31, 2019, 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.

Since January 1, 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 one unmet commercial milestone 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, 2020, for option 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 in the three months ended March 31, 2020.

At March 31, 2020, 457,602 performance-based stock options were both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $34.1 million.

Stock-based compensation expense recognized during the three months ended March 31, 2020 and 2019 was as follows:

	Three Months Ended March 31,
	2020		2019
	(in thousands)
Research and development	$	12,224	$	20,745
Selling, general and administrative		18,886		23,371
	$	31,110	$	44,116

Stock-based compensation expense recognized during the three months ended March 31, 2020 and 2019 by award type was as follows:

	Three Months Ended March 31,
	2020		2019
	(in thousands)
Stock options	$	30,218	$	43,564
Restricted stock units		—		58
Employee stock purchase plan		892		494
	$	31,110	$	44,116

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, 2020 and 2019 was $46.44 and $98.23, respectively.

The table below summarizes activity related to time-based and performance-based stock options:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2019		8,163,113		$	106.30		7.75	$	87,972
Granted		770,860			68.90
Exercised		(7,196	)		50.99
Forfeited		(314,812	)		128.21
Outstanding as of March 31, 2020		8,611,965			102.20		7.69	$	7,083
Exercisable as of March 31, 2020		4,253,664			85.18		6.63	$	7,083

At March 31, 2020, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $264.0 million, which is expected to be recognized over the remaining weighted average vesting period of 2.34 years.

The intrinsic value of stock options exercised during the three months ended March 31, 2020 and 2019 was $0.1 million and $26.7 million, respectively.

As of March 31, 2020, the total number of shares reserved under all equity plans was 12,685,993 and the total number of shares available for future issuance under all equity plans was 3,369,397 shares.