In June 2014, as a component of our public offering, we sold 600,000 pre-funded warrants to purchase shares of our common stock. The pre-funded warrants have an exercise price of $0.01 per share and expired on June 30, 2021. During the three months ended June 30, 2021, warrant holders exercised 195,574 warrants using the cashless exercise provision, which resulted in the issuance of 195,461 shares and no cash proceeds. As of June 30, 2021, no warrants from the June 2014 public offering remain outstanding.

The following table summarizes the stock option activity for the six months ended June 30, 2021:

	Shares (in thousands)			Weighted- average Exercise Price		Weighted- average Remaining Contractual Term (Years)
Outstanding at December 31, 2020		18,912		$	19.59		7.21
Granted		628		$	15.87
Exercised		(358	)	$	15.51
Expired and forfeited		(875	)	$	22.50
Outstanding at June 30, 2021		18,307		$	19.40		7.02

The following table summarizes the restricted stock unit activity for the six months ended June 30, 2021:

	Shares (in thousands)			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2020		603		$	15.84
Granted		150		$	15.29
Released		(77	)	$	15.98
Expired and forfeited		(22	)	$	16.06
Outstanding at June 30, 2021		654		$	15.69

For the six months ended June 30, 2021, 357,839 shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of $5.5 million.

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,
	2021		2020		2021		2020
Research and development	$	4,672	$	4,681	$	9,609	$	9,968
General and administrative		2,792		3,304		5,578		6,735
Sales and marketing		3,777		3,129		7,540		6,385
Total stock-based compensation expense	$	11,241	$	11,114	$	22,727	$	23,088

As of June 30, 2021, there was $97.6 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 2.6 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,
	2021			2020
Risk-free interest rate		1.0	%		1.1	%
Dividend yield		0.0	%		0.0	%
Volatility		66.6	%		66.7	%
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,
	2021			2020
Risk-free interest rate		0.04	%		0.10	%
Dividend yield		0.0	%		0.0	%
Volatility		48.3	%		63.3	%
Expected life (months)		6			6