In 1997, our stockholders approved our Employee Stock Purchase Plan (“ESPP”) at which time a maximum of 10,000 shares of common stock were available for issuance. In December 2007, May 2009, June 2011, May 2014, May 2015, June 2016, June 2017, June 2019 and June 2021, our stockholders authorized increases in the number of shares reserved for issuance under the ESPP by 5,000, 10,000, 25,000, 25,000, 100,000, 100,000, 200,000, 300,000 and 200,000 shares, respectively, for a total of 975,000 shares reserved at December 31, 2021. Under the terms of the ESPP, employees can elect to have up to a maximum of 10% of their base earnings withheld to purchase shares of our common stock. The purchase price of the stock is 85% of the lower of the closing prices for our common stock on either: (i) the first trading day in the enrollment period, as defined in the ESPP, in which the purchase is made, or (ii) the purchase date. The length of the enrollment period is 6 months. Enrollment dates are the first business day of May and November. Under the ESPP, we issued 175,228, 193,841, and 125,727 shares in 2021, 2020 and 2019, respectively. The weighted-average exercise price per share of the purchase rights exercised during 2021, 2020 and 2019 was $12.21, $11.95 and $16.77, respectively. As of December 31, 2021, 773,978 shares of common stock have been issued under the ESPP and 201,022 shares of common stock are available for future issuance.

We currently have one stock option plan from which we can grant options and restricted stock awards to employees, officers, directors and consultants. In December 2007, the stockholders approved our 2007 Amended and Restated Equity Incentive Plan (“2007 Plan”) at which time a maximum of 150,000 shares of common stock were available for grant. In May 2010, June 2011, May 2014, May 2015, June 2016, June 2017, June 2019 and June 2021, our stockholders approved amendments to our 2007 Plan to increase the maximum number of shares of common stock available for grant by 100,000, 4,500,000, 1,750,000, 4,300,000, 3,000,000, 5,000,000, 7,000,000 and 2,000,000 shares of common stock, respectively, resulting in an aggregate of 27,800,000 shares of common stock authorized for issuance as of December 31, 2021. At December 31, 2021, there were 868,880 shares available for future grant under the 2007 Plan. Any shares that are issuable on exercise of options granted that expire, are cancelled or that we receive pursuant to a net exercise of options are available for future grant and issuance.

In 2014, 2013 and 2012, we granted options to certain employees outside of our stockholder approved stock option plans. All options to purchase our common stock were granted with an exercise price that equals fair market value of the underlying common stock on the grant dates and expire no later than 10 years from the date of grant. The options are exercisable in accordance with vesting schedules that generally provide for them to be fully vested and exercisable 4 years after the date of grant, provided, however, that we have also issued stock options awards that are subject to performance vesting requirements. All stock option grants issued outside of our stockholder approved plans have been registered on Form S-8 with the SEC.

In 2020, we began granting restricted stock units (“RSUs”) to employees and non-employee directors pursuant to the 2007 Plan. We satisfy such grants through the issuance of new shares upon vesting.

	Outstanding Options
				Weighted-
				Average
	Number of			Exercise
	Shares			Price
Balance at December 31, 2020		18,912,529		$	19.59
Granted		2,053,980		$	12.39
Exercised		(369,978	)	$	15.29
Cancelled		(1,652,594	)	$	21.69
Balance at December 31, 2021		18,943,937		$	18.71



	Outstanding RSUs
				Weighted-
				Average
	Number of			Grant Date
	Shares			Fair Value
Balance at December 31, 2020		602,741		$	15.84
Granted		2,448,957		$	10.86
Released		(154,487	)	$	15.98
Forfeited		(94,018	)	$	15.07
Balance at December 31, 2021		2,803,193		$	11.51

For the year ended December 31, 2021, equity awards cancelled (included in options outstanding) consisted of 806,244 options forfeited with a weighted-average exercise price of $20.51 and 846,350 options expired with a weighted-average exercise price of $22.81.

As of December 31, 2021, options exercisable have a weighted-average remaining contractual term of 6.6 years. The total intrinsic value of stock option exercises, which is the difference between the exercise price and closing price of our common stock on the date of exercise, during the years ended December 31, 2021 and 2020 was $0.7 million and $3.2 million, respectively. As of December 31, 2021 and 2020, the total intrinsic value of options outstanding and exercisable was $1.7 million and $69.1 million, respectively.

	Years Ended December 31,
	2021				2020				2019
			Weighted-				Weighted-				Weighted-
			Average				Average				Average
			Exercise				Exercise				Exercise
	Options		Price		Options		Price		Options		Price
Exercisable at end of year		12,243,887	$	19.13		10,237,202	$	18.74		7,436,379	$	17.02
Options vested or expected to vest		18,416,243	$	18.77		18,179,553	$	19.58		15,962,432	$	20.31

Exercise prices and weighted-average remaining contractual lives for the options outstanding as of December 31, 2021 were:

									Weighted-
			Weighted-						Average
			Average		Weighted-				Exercise
			Remaining		Average				Price of
Outstanding		Range of	Contractual		Exercise		Options		Options
Options		Exercise Prices	Life (in years)		Price		Exercisable		Exercisable
	2,697,794	$7.20–$10.55		5.28	$	9.09		1,598,055	$	8.17
	2,266,285	$10.64–$15.15		5.84		13.40		1,663,593		13.39
	3,307,235	$15.22-$15.72		8.54		15.71		1,035,174		15.70
	2,824,254	$15.75–$18.18		5.73		16.94		2,518,197		16.91
	3,045,374	$18.26–$24.97		6.56		23.04		2,276,351		23.17
	3,102,637	$25.02		7.76		25.02		1,619,032		25.02
	1,700,358	$25.06–$39.00		5.23		30.53		1,533,485		30.52
	18,943,937			6.59		18.71		12,243,887		19.13

On December 31, 2021, we had reserved 21,747,130 shares of common stock for future issuance on exercise of outstanding options and vesting of outstanding restricted stock units granted under the 2007 Plan, as well as the non-plan grants.

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

	December 31,
	2021		2020		2019
Research and development	$	19,482	$	20,731	$	19,202
General and administrative		11,816		15,601		13,564
Sales and marketing		15,553		13,886		18,645
Total stock-based compensation expense	$	46,851	$	50,218	$	51,411

As of December 31, 2021, there was $99.7 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 2.8 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 and ESPP purchase right on the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:

	December 31,
	2021			2020			2019
Risk-free interest rate		1.1	%		0.5	%		1.8	%
Dividend yield		—	%		—	%		—	%
Volatility		62.6	%		69.4	%		66.5	%
Expected life (years)		6			6			6

	December 31,
	2021			2020			2019
Risk-free interest rate		0.1	%		0.1	%		1.9	%
Dividend yield		—	%		—	%		—	%
Volatility		46.9	%		67.7	%		52.1	%
Expected life (months)		6			6			6

The weighted-average fair value of options granted was $7.10, $9.76 and $14.70 for the years ended December 31, 2021, 2020 and 2019, respectively.

The weighted-average fair value of shares purchased through the ESPP was $3.64, $5.18 and $5.94 for the years ended December 31, 2021, 2020 and 2019, respectively.

The risk-free interest rate assumption is based on observed interest rates on U.S. Treasury debt securities with maturities close to the expected term of our employee and director stock options and ESPP purchases.

The dividend yield assumption is based on our history and expectation of dividend payouts. We have never paid dividends on our common stock, and we do not anticipate paying dividends in the foreseeable future.

The expected life of employee and director stock options represents the average of the contractual term of the options and the weighted-average vesting period, as permitted under the simplified method. We have elected to use the simplified method, as we do not have enough historical exercise experience to provide a reasonable basis on which to estimate the expected term. The expected life for the ESPP purchase rights is 6 months, which represents the length of each purchase period.