In January 2021, the Board of Directors authorized an additional 1,499,037 shares to be issued under the 2015 Equity Incentive Plan (the 2015 Plan). As of March 31, 2021, the total number of shares of common stock authorized for issuance under the 2015 Plan and the 2014 Stock Plan (the 2014 Plan) was 13,911,954, of which 2,447,174 remained available for future grants under the 2015 Plan.

The Company’s stock-based compensation expense by award type was as follows (in thousands):

	Three Months Ended March 31,
	2021		2020
Stock options	$	9,015	$	7,779
Restricted stock units		679		—
Employee stock purchase plan		226		238
	$	9,920	$	8,017

As of March 31, 2021, the Company had $93.8 million of unrecognized stock-based compensation expense related to stock options, restricted stock units and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.8 years.

The Company recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive loss as follows (in thousands):

The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data):

						Weighted-
						average
				Weighted-		Remaining
				average		Contractual		Aggregate
				Exercise		Life		Intrinsic
	Shares			Price		(Years)		Value (a)
Outstanding at December 31, 2020		6,361		$	31.21		7.2	$	101,356
Granted		1,170		$	44.92
Exercised		(105	)	$	12.30
Cancelled or forfeited		(80	)	$	45.48
Outstanding at March 31, 2021		7,346		$	33.51		7.4	$	51,356
Exercisable at March 31, 2021		4,126		$	26.47		6.2	$	50,065
Vested and expected to vest at March 31, 2021		7,346		$	33.51		7.4	$	51,356

The weighted-average grant date fair value per share of options granted during the three months ended March 31, 2021 was $27.28. During the three months ended March 31, 2021, the total number of stock options exercised was 104,638, resulting in total proceeds of $1.3 million. The total intrinsic value of options exercised during the three months ended March 31, 2021 was $3.6 million.

The following table summarizes restricted stock unit activity under the 2015 Plan (in thousands, except per share data):

				Weighted-
				average
				Grant Date
	Shares			Fair Value
Unvested balance at December 31, 2020		—		$	—
Granted		257		$	44.92
Vested		—		$	—
Forfeited		(3	)	$	44.97
Unvested balance at March 31, 2021		254		$	44.92

No restricted stock units vested during the three months ended March 31, 2021 and 2020.

In January 2021, the Board of Directors authorized an additional 374,759 shares to be issued under the 2015 ESPP. As of March 31, 2021, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 998,683, of which 803,728 remained available for future issuance. During the three months ended March 31, 2021, 19,042 shares of common stock were issued under the 2015 ESPP.