The following table summarizes the stock option activity for the six months ended June 30, 2022:

	Shares (in thousands)			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)
Outstanding at December 31, 2021		18,944		$	18.71		6.59
Granted		2,629		$	3.27
Exercised		—		$	—
Expired and forfeited		(1,202	)	$	20.62
Outstanding at June 30, 2022		20,371		$	16.60		6.59

The following table summarizes the restricted stock unit activity (“RSUs”) for the six months ended June 30, 2022:

	Shares (in thousands)			Weighted-Average Grant Date Fair Value
Outstanding at December 31, 2021		2,803		$	11.51
Granted		27		$	5.23
Released		(449	)	$	11.61
Expired and forfeited		(174	)	$	11.53
Outstanding at June 30, 2022		2,207		$	11.41

The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
Research and development	$	4,422	$	4,672	$	8,985	$	9,609
General and administrative		2,783		2,792		5,785		5,578
Sales and marketing		3,148		3,777		6,498		7,540
Total stock-based compensation expense	$	10,353	$	11,241	$	21,268	$	22,727

As of June 30, 2022, there was $77.3 million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 2.3 years.The fair value of RSUs is estimated based on the closing market price of our common stock on the date of the grant. RSUs generally vest quarterly over a four-year period.

We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:

	June 30,
	2022			2021
Risk-free interest rate		3.1	%		1.0	%
Dividend yield		0.0	%		0.0	%
Volatility		60.8	%		66.6	%
Expected life (years)		6			6

We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan, as amended, at the beginning of each new offering period using the Black-Scholes option pricing model with the following weighted-average assumptions:

	June 30,
	2022			2021
Risk-free interest rate		1.5	%		0.04	%
Dividend yield		0.0	%		0.0	%
Volatility		75.3	%		48.3	%
Expected life (months)		6			6