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Stock Plans and Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

A summary of the Company’s option activity and related information for the nine months ended September 30, 2020 is as follows:

	Number of Shares			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (Years)		Total Intrinsic Value (in thousands)
Outstanding as of December 31, 2019		5,613,353		$	15.90
Granted		1,486,780			16.79
Exercised		(172,779	)		9.78
Forfeited		(266,436	)		21.49
Outstanding as of September 30, 2020		6,660,918		$	16.03		7.1	$	26,951
Options vested and exercisable as of September 30, 2020		3,652,897		$	14.34		5.6	$	22,820

Schedule of RSU's and Related Information

A summary of the Company’s RSUs and related information for the nine months ended September 30, 2020 is as follows:

	Number of RSUs				Weighted Average Fair Value Per RSU at Grant Price
Nonvested as of December 31, 2019		758,718			$	25.47
Granted		531,164				16.31
Vested		(228,925	)			24.73
Forfeited		(69,368	)			23.68
Nonvested as of September 30, 2020		991,589		⁽¹⁾	$	20.86

⁽¹⁾	Includes 140,000 RSUs that have vested but are subject to deferred settlement, which have a weighted average remaining contractual term of 2.4 years.

Assumptions for Weighted Average Fair Value of Options Granted

The fair value of the stock options granted or modified during the periods indicated was estimated using the Black-Scholes option pricing model, based on the following assumptions:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2020	2019	2020	2019
Exercise price	$20.14 - $23.30	$10.91 - $49.14	$14.45 - $23.30	$10.91 - $57.33
Expected volatility	68.9% - 81.6%	66.4% - 80.6%	66.4% - 82.2%	66.2% - 83.2%
Risk-free rate	0.26% - 0.52%	1.36% - 1.89%	0.26% - 1.44%	1.36% - 2.65%
Expected term (years)	5.5 - 7.0	5.5 - 9.6	5.5 - 7.5	5.0 - 9.6
Expected dividend yield	0%	0%	0%	0%

Schedule of Stock-Based Compensation Expense

The following table summarizes the components of total stock-based compensation expense included in the condensed consolidated statements of operations and comprehensive loss (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Research and development	$	2,759	$	2,464	$	8,310	$	8,309
General and administrative		3,289		2,940		9,796		5,740	⁽¹⁾
Total stock-based compensation expense	$	6,048	$	5,404	$	18,106	$	14,049

⁽¹⁾	^{Includes the reversal of previously recognized stock-based compensation expense of $3.6 million related to forfeited awards resulting from the departure of one of the Company’s former officers during the period.}