As of June 30, 2021, the Company had an active stock-based incentive compensation plan, an employee stock purchase plan and an equity inducement plan: the 2015 Equity Incentive Award Plan (as amended and restated, the “2015 Plan”), the 2015 Employee Stock Purchase Plan (as amended and restated, the “ESPP”), and the 2018 Equity Inducement Award Program (the “2018 Plan”), respectively. All new equity compensation grants are issued under these three plans; however, outstanding awards previously issued under inactive plans will continue to vest and remain exercisable in accordance with the terms of the respective plans.

The 2015 Plan and the 2018 Plan provide for the grant of stock and stock-based awards including stock options, restricted stock units (including deferred stock units), performance-based stock units, and stock appreciation rights. As of June 30, 2021, there were 5.6 million shares available for grant under the 2015 Plan and 2018 Plan.

Total stock-based compensation expense recognized in the Company’s consolidated statements of operations and comprehensive loss is classified as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Cost of revenue	$	397	$	270	$	632	$	508
Research and development		675		636		1,311		1,156
Selling and marketing		454		343		748		559
General and administrative		4,929		4,554		12,258		9,081
Total stock-based compensation expense	$	6,455	$	5,803	$	14,949	$	11,304

The Company’s stock-based compensation expense is based on the value of the portion of share-based payment awards that are ultimately expected to vest, assuming estimated forfeitures at the time of grant. Stock-based compensation relating to stock-based awards granted to consultants was insignificant for the six months ended June 30, 2021 and 2020.

Restricted Stock Units, Deferred Stock Units and Performance Share Units (collectively “Incentive Stock Units” or “ISUs”)

The Company grants Restricted Stock Units (“RSUs”), to its board of directors and employees for their services. Additionally, the Company grants Deferred Stock Units (“DSUs”), to its board of directors at their election in lieu of retainer and committee service fees.

The DSUs granted to board members are either fully vested upon issuance or vest over a period of time from the grant date and will be released and settled upon termination of the board member’s services, the occurrence of a change in control event, or the tenth anniversary of the grant date.

The RSUs are generally granted with a grant date fair value equal to the market price of our stock on the date of grant and generally vest in equal annual or monthly installments over either two or three years from the grant date and are subject to the participants

continuing service to the Company over that period. The weighted-average grant date fair value of RSUs granted six months ended June 30, 2021 and 2020 was $4.78 per share, and $2.78 per share, respectively.

In March 2021, the Company introduced a performance share plan (the “2021 PSU Plan”) as a component of its equity grants for 2021. The 2021 PSU Plan provides for the award of performance share units (“PSUs”) to employees which will be awarded based on the achievement of performance targets in the Company’s compound annual revenue growth rate over a three-year period.

The table below summarizes the Company’s activity and related information for its ISUs:

	ISUs
	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2020		8,046,399		$	3.41
ISUs granted		3,178,689		$	4.78
ISUs vested		(3,509,497	)	$	4.82
ISUs forfeited		(178,503	)	$	3.34
Unvested at June 30, 2021		7,537,088		$	3.99
Vested and unreleased		172,692
Outstanding at June 30, 2021		7,709,780

The total grant date fair value of ISUs awarded was $15.1 million and $16.4 million for the six months ended June 30, 2021 and 2020, respectively. The total fair value of ISUs vested was $18.3 million, and $1.2 million during the six months ended June 30, 2021 and 2020, respectively.

At June 30, 2021, total unrecognized stock-based compensation cost related to ISUs, net of estimated forfeitures, was $19.1 million, which is expected to be recognized over a weighted-average period of 1.9 years. As of June 30, 2021, 7.0 million shares of ISUs are expected to vest.

Stock options awards are generally granted with an exercise price equal to the market price of the Company’s common stock at the date of grant and with a four-year vesting schedule. Stock option awards generally expire 10 years from the date of grant.

A summary of the Company’s stock option activity and related information is as follows:

	Number of Stock Options Outstanding			Weighted-Average Exercise Price		Weighted- Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
								(In thousands)
Options outstanding at December 31, 2020		8,142,348		$	7.14		7.3	$	2,638
Options granted		—			—
Options exercised		(43,651	)	$	4.26
Options cancelled or forfeited		(680,493	)	$	9.20
Options outstanding at June 30, 2021		7,418,204		$	6.97		6.7	$	7,905
Options exercisable at June 30, 2021		5,452,385		$	6.94		6.3	$	5,559
Options vested and expected to vest at June 30, 2021		7,260,596		$	7.00		6.6	$	7,573

There were no options granted to employees for the six months ended June 30, 2021. The weighted-average grant date fair value of options granted to employees was $1.20 per share for the six months ended June 30, 2020.

Aggregate intrinsic value represents the difference between the estimated fair value of the underlying common stock and the exercise price of outstanding, in-the-money options. The aggregate intrinsic value of options exercised was nominal for the six months ended June 30, 2021 and 2020.

At June 30, 2021, total unrecognized stock-based compensation cost related to stock options granted to employees, net of estimated forfeitures, was $7.2 million, which is expected to be recognized over a weighted-average period of 1.6 years.

The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the estimated fair value of the Company’s common stock, as well as assumptions regarding a number of complex and subjective variables. The variables used to calculate the fair value of stock options using the Black-Scholes option-pricing model include actual and projected employee stock option exercise behaviors, expected price volatility of the Company’s common stock, the risk-free interest rate and expected dividends. Each of these inputs is subjective and generally requires significant judgment to determine.

The risk-free interest rate is based on the zero-coupon U.S. Treasury notes, with maturities similar to the expected term of the options. The Company has not paid and does not anticipate paying cash dividends on its common stock; therefore, the expected dividend yield is assumed to be zero.

During the fourth quarter of 2020, the Company began to determine volatility by solely using the Company’s own historical volatility measurements, since more than four years of historical data became available in the public market. Prior to the fourth quarter of 2020, the Company determined the volatility for stock options granted based on the average historical price volatility for the Company and industry peers over a period equivalent to the expected term of the stock option grants.

The forfeiture rate of stock options is estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. Forfeitures have been estimated by the Company based upon historical and expected forfeiture experience.

The fair value of employee stock options was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:

In July 2015, the Company adopted the ESPP. Certain employees, as defined by the ESPP, are eligible to participate in the ESPP if employed by the Company for at least 20 hours per week during at least five months per calendar year. Participating employees may contribute up to the lesser of 15% of their eligible earnings or $30,000 during each offering period, provided that in no event shall a participating employee be permitted to purchase more than 3,000 shares of common stock during each offering period.

During 2021, the first offering period provided to eligible employees is January 1, 2021 through June 30, 2021. The purchase price of common stock purchased under the ESPP is currently equal to 85% of the lesser of the fair market value of a share of common stock on: (1) the first trading day of an offering period and (2) the last trading of each offering period. At June 30, 2021, 3.5 million shares were reserved for issuance under the ESPP. No more than 3.5 million shares of common stock may be issued under the ESPP. As of June 30, 2021, 0.3 million shares have been issued under the ESPP and 3.2 million shares remained available for future issuance under the ESPP. Purchase rights granted under the ESPP are valued using the Black-Scholes pricing model.