The Company maintains an Employee Stock Purchase Plan (the “Purchase Plan”), which is intended to qualify as an employee stock purchase plan within the meaning of Section 423(b) of the Internal Revenue Code. Under the Purchase Plan, the Company’s Board of Directors may authorize participation by eligible employees, including officers, in periodic offerings. Under the Purchase Plan eligible employee participants may purchase shares of common stock of the Company at a purchase price equal to 85% of the lower of the fair market value per share on the start date of the offering period or the fair market value per share on the purchase date. The Purchase Plan consists of a fixed offering period of 12 months with two purchase periods within each offering period. In June 2020, the Company’s stockholders approved an amendment and restatement of the Purchase Plan that increased the aggregate number of shares of common stock authorized for issuance under the Purchase Plan by 1.5 million shares. At December 31, 2020, the Company had 1.8 million shares available for future issuance.

The Company also maintains an equity compensation plan to provide long-term incentives for employees, contractors, and members of its Board of Directors. The Company currently grants equity awards from one plan, the 2008 Equity Incentive Plan and its subsequent amendments (collectively, the “Amended 2008 Plan”). The Amended 2008 Plan allows for the issuance of non-statutory and incentive stock options, restricted stock, restricted stock units (“RSUs”), stock appreciation rights, other stock-related awards, and performance awards which may be settled in cash, stock, or other property. In June 2019, the Company’s stockholders approved an amendment and restatement of the Amended 2008 Plan that increased the aggregate number of shares of common stock authorized for issuance by 11.8 million shares. In June 2020, the Company’s stockholders approved an amendment and restatement of the Amended 2008 Plan that increased the aggregate number of shares of common stock authorized for issuance by 5.0 million shares. Option

awards under the Amended 2008 Plan generally have a maximum term of 10 years from the date of the award. The Amended 2008 Plan generally requires options to be granted at 100% of the fair market value of the Company’s common stock subject to the option on the date of grant. Options granted by the Company to employees generally vest over four years. RSUs are measured based on the fair market value of the underlying stock on the date of grant. RSUs granted by the Company to employees generally vest over three to four years. Performance-based stock or cash awards granted under the Amended 2008 Plan are limited to either 500,000 shares of common stock or $1.0 million per recipient per calendar year. At December 31, 2020, 20,000 performance-based stock awards were outstanding.

At December 31, 2020, the Company had an aggregate of approximately 30.5 million shares of its common stock subject to outstanding options or unvested RSUs, or remaining available for future issuance under the Amended 2008 Plan, of which approximately 16.3 million shares and 5.7 million shares were subject to outstanding options and unvested RSUs, respectively, and approximately 8.4 million shares were available for future issuance under the Amended 2008 Plan. The Company’s policy is to issue new shares of common stock upon the exercise of options or vesting of RSUs.

Activity under the Company’s equity incentive plans related to stock options is set forth below (in thousands except per share amounts):

	Number of Options Outstanding			Weighted Average Exercise Price per Share
Balances at December 31, 2019		16,830		$	4.53
Granted		2,237			5.22
Exercised		(2,658	)		4.01
Forfeited/canceled		(103	)		5.27
Balances at December 31, 2020		16,306			4.71

Activity under the Company’s equity incentive plans related to RSUs is set forth below (in thousands except per share amounts):

	Number of RSUs Unvested			Weighted Average Grant Date Fair Value per Share
Balances at December 31, 2019		4,098		$	5.24
Granted ⁽¹⁾		3,433			5.17
Vested ⁽¹⁾		(1,631	)		5.05
Forfeited⁽¹⁾		(161	)		5.73
Balances at December 31, 2020		5,739			5.24

The total fair value of RSUs as of their respective vesting dates, for the years ended December 31, 2020, 2019 and 2018, were $6.8 million, $5.6 million and $2.8 million, respectively.

Information regarding the Company’s stock options outstanding, stock options vested and expected to vest, and stock options exercisable at December 31, 2020, was as follows (in thousands except weighted average exercise price and contractual term):

	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Balances at December 31, 2020
Stock options outstanding		16,306	$	4.71		5.26	$	36,180
Stock options vested and expected to vest		16,151		4.70		5.22		35,905
Stock options exercisable		12,735		4.61		4.36		29,535

The aggregate intrinsic value in the table above is calculated as the difference between the exercise price of the stock option and the Company’s closing stock price on the last trading day of each respective fiscal period.

The total intrinsic value of options exercised for the years ended December 31, 2020, 2019 and 2018, was $7.9 million, $1.6 million and $7.1 million, respectively. The total intrinsic value of exercised stock options is calculated based on the difference between the exercise price and the quoted market price of the Company’s common stock as of the close of the exercise date.

Stock-based compensation expense recognized on the Company’s consolidated statements of operations for the years ended December 31, 2020, 2019 and 2018, was as follows (in thousands):

	Year Ended December 31,
	2020		2019		2018
Research and development	$	3,739	$	2,472	$	1,669
Selling, general and administrative		14,290		10,840		8,725
Total stock-based compensation expense	$	18,029	$	13,312	$	10,394

Stock-based compensation expense in the above table does not reflect any income taxes as the Company has experienced a history of net losses since its inception and has a nearly full valuation allowance on its deferred tax assets. In addition, there was neither income tax benefits realized related to stock-based compensation expense nor any stock-based compensation costs capitalized as part of an asset during the years ended December 31, 2020, 2019 and 2018. The Company has also not recorded any stock-based compensation associated with performance-based stock options during the years ended.

As of December 31, 2020, the Company expects to recognize the remaining unamortized stock-based compensation expense of $7.0 million and $18.2 million, respectively, related to non-vested stock options and RSUs, net of estimated forfeitures, over an estimated remaining weighted average period of 2.3 years and 1.8 years, respectively.

The Company uses the Black-Scholes option pricing model to determine the grant-date fair value of stock options and employee stock purchase plan rights. The Black-Scholes option-pricing model is affected by the Company’s stock price, as well as assumptions regarding a number of complex and subjective variables, which include the expected term of the grants, actual and projected employee stock option exercise behaviors, including forfeitures, the Company’s expected stock price volatility, the risk-free interest rate and expected dividends. The Company recognizes the grant-date fair value of the stock award as stock-based compensation expense on a straight-line basis over the requisite service period, which is the vesting period, and is adjusted for estimated forfeitures.

The expected life of the stock options is based on observed historical exercise patterns. Groups of employees having similar historical exercise behavior are considered separately for valuation purposes. The Company estimates stock option forfeitures based on historical data for employee groups. The total number of stock options expected to vest is adjusted by actual and estimated forfeitures.

The expected volatility is estimated by using historical volatility of the Company’s common stock. The risk-free interest rate is based on the implied yield on a U.S. Treasury zero-coupon issue with a remaining term commensurate with the expected term of the option. The Company does not anticipate paying any cash dividends in the foreseeable future and therefore uses an expected dividend yield of zero.

The weighted average assumptions used to value the Company’s stock-based awards for the years ended December 31, 2020, 2019 and 2018, was as follows:

	Year Ended December 31,
	2020		2019		2018
Stock Options:
Expected term (in years)		6.89		5.59		6.07
Estimated volatility	52%		50%		50%
Risk-free interest rate	0.94%		2.32%		2.72%
Expected dividend yield	0%		0%		0%
Employee Stock Purchase Plan Rights:
Expected term (in years)		0.72		0.80		0.74
Estimated volatility	53%		46%		47%
Risk-free interest rate	0.86%		2.04%		2.34%
Expected dividend yield	0%		0%		0%

The weighted average grant-date fair value of stock options granted during the years ended December 31, 2020, 2019 and 2018, was $2.71 per share, $2.73 per share and $2.41 per share, respectively. The weighted average grant-date fair value of employee stock purchase rights during the years ended December 31, 2020, 2019 and 2018, was $1.71 per share, $1.84 per share and $2.29 per share, respectively.