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 expire on June 30, 2021. As of March 31, 2021, 195,574 warrants from the June 2014 public offering remain outstanding.

The following table summarizes the stock option activity for the three months ended March 31, 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		102		$	18.96
Exercised		(71	)	$	15.86
Expired and forfeited		(148	)	$	20.81
Outstanding at March 31, 2021		18,795		$	19.59		6.92

The following table summarizes the restricted stock unit activity for the three months ended March 31, 2021:

	Shares (in thousands)			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2020		603		$	15.84
Granted		17		$	18.93
Released		(39	)	$	15.98
Expired and forfeited		(11	)	$	15.85
Outstanding at March 31, 2021		570		$	15.93

For the three months ended March 31, 2021, 70,885 shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of $1.1 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 March 31,
	2021		2020
Research and development	$	4,937	$	5,287
General and administrative		2,786		3,431
Sales and marketing		3,763		3,256
Total stock-based compensation expense	$	11,486	$	11,974

As of March 31, 2021, there was $104.4 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 2.7 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:

	March 31,
	2021			2020
Risk-free interest rate		0.7	%		1.3	%
Dividend yield		0.0	%		0.0	%
Volatility		69.4	%		66.2	%
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. There were no new offering periods for the three months ended March 31, 2021 and 2020.