Summary of Employee and Non-Employee Stock-Based Compensation Expense

The following table summarizes employee and non-employee stock-based compensation expense for the three months ended March 31, 2022 and 2021 and the allocation within the condensed consolidated statements of operations and comprehensive loss (in thousands):


	March 31,
	2022		2021
General and administrative expense	$	4,431	$	3,478
Research and development expense		4,809		3,513
Total stock-based compensation	$	9,240	$	6,991

Activity for service-based stock options under the 2019 Plan is as follows:


	Number of Options and Unvested Shares			Weighted Average Exercise Price		Weighted average remaining contractual life (years)		Aggregate intrinsic value
								(In thousands)
Balance at December 31, 2021		5,571,293		$	28.70	8.26		$	15,687
Options granted		1,087,058			13.22
Options exercised		(23,389	)		0.58
Options cancelled / forfeited		(411,564	)		36.68
Balance at March 31, 2022		6,223,398		$	25.58		8.16	$	5,225
Options vested and expected to vest as of March 31, 2022		6,223,398			25.58		8.16		5,225
Options exercisable as of March 31, 2022		2,557,002		$	20.81		7.01	$	4,513

Summary of Stock Options Activity

The following table summarizes activity under the Company’s PSOs from the 2019 Plan and related information:


	Shares Subject to Outstanding PSOs			Weighted Average Exercise Price		Weighted average remaining contractual life (years)
Outstanding balance at December 31, 2021		675,000		$	29.60		8.94
Surrendered		(400,000	)	$	29.60	—
Vested	—			—		—
Outstanding balance at March 31, 2022		275,000		$	29.60		8.70