Our board of directors sets the terms, conditions, and restrictions related to our 2017 Employee Stock Purchase Plan (ESPP) and the grant of stock options, restricted stock units (RSUs) and performance-based awards under our stock-based plans. Our board of directors determines the number of awards to grant and also sets vesting criteria.

In general, RSUs vest on a quarterly basis over a period of four years from the date of grant at a rate of 25% on the first anniversary of the grant date and ratably each quarter over the remaining 12 quarters, or ratably over 16 quarters, subject to the employees’ continued employment. We may grant RSUs with different vesting terms from time to time.

Incentive stock options and non-statutory stock options granted under the 2011 Equity Incentive Plan (2011 Plan) have a term of no more than ten years from the date of grant and an exercise price of at least 100% of the fair market value of the underlying common stock on the date of grant. Generally, options vest at a rate of either 25% on the first anniversary of the option grant date and ratably each month over the remaining period of 36 months, or ratably each month over 48 months. We may grant options with different vesting terms from time to time.

For performance-based share awards, our board of directors sets the performance objectives and other vesting provisions in determining the number of shares or value of performance units and performance shares that will be paid out. Such payout will be a function of the extent to which performance objectives or other vesting provisions have been achieved.

In January 2011, our board of directors adopted the 2011 Plan under which incentive stock options, non-statutory stock options, RSUs, stock appreciation rights, performance stock units (PSUs), and performance shares may be granted to our employees, directors, and consultants. In April 2019, our board of directors authorized, and in June 2019, our stockholders approved, an amendment and restatement of the 2011 Plan to make various changes, including increasing the number of shares reserved for issuance by approximately 5.0 million shares and extending the term of the 2011 Plan until April 2029. In May 2020, our board of directors authorized, and in June 2020, our stockholders approved, an increase of 1.4 million in the number of shares reserved for issuance under the 2011 Plan. In April 2021, our board of directors authorized, and in May 2021, our stockholders approved, an additional increase of 4.1 million in the number of shares reserved for issuance under the 2011 Plan.


	Year Ended December 31,
	2021			2020			2019
Stock options
Weighted average expected volatility	94.0		%	79.0		%	69.5		%
Weighted average expected term	4.2 years			3.8 years			4.3 years
Weighted average risk-free interest rate	0.6		%	2.6		%	1.9		%
Dividend yield	—			—			—
Weighted-average fair value per share	$	3.73		$	2.60		$	7.17


	Number of Units (in 000s)	Weighted-Average Grant Date Fair Value per Unit
Balance at December 31, 2018	1,812	$	7.09
RSU granted	1,808	$	8.08
RSU released	(730)	$	8.06
RSU forfeited	(339)	$	7.80
Balance at December 31, 2019	2,551	$	7.43
RSU granted	3,788	$	4.06
RSU released	(1,139)	$	7.04
RSU forfeited	(338)	$	6.24
Balance at December 31, 2020	4,862	$	4.98
RSU granted	3,295	$	5.23
RSU released	(2,225)	$	5.02
RSU forfeited	(791)	$	4.66
Balance at December 31, 2021	5,141	$	5.18

The total intrinsic value of RSUs vested and released during the year ended December 31, 2021, 2020 and 2019 were approximately $11.2 million, $8.0 million and $5.8 million, respectively. The intrinsic value of vested and released RSUs is calculated by multiplying the fair market value of our common stock on the vesting date by the number of shares vested. As of

December 31, 2021, the unrecognized compensation costs related to outstanding unvested RSUs under our equity incentive plans were $22.5 million. We expect to recognize those costs over a weighted average period of 2.5 years.


	Number of Options (in 000s)	Weighted-Average Exercise Price per Option		Weighted- Average Remaining Contractual Life (in Years)	Aggregate Intrinsic Value(1) in (000s)
Balance at December 31, 2018	2,385	$	7.56	7.8
Options granted	50	$	13.08
Options exercised	(197)	$	5.43		$	1,198
Options forfeited	(211)	$	8.73
Balance at December 31, 2019	2,027	$	7.78	6.8
Options granted	117	$	4.05
Options exercised	(100)	$	4.84		$	359
Option forfeited	(409)	$	9.22
Balance at December 31, 2020	1,635	$	7.33	6.2	$	834
Options granted	92	$	5.56
Options exercised	(37)	$	5.62		$	25
Options forfeited	(93)	$	10.49
Balance at December 31, 2021	1,597	$	7.08	5.6	$	82
Vested at December 31, 2021	1,503	$	7.13	5.5	$	82
Unvested awards at December 31, 2021	94	$	6.30	8.0	$	—

(1)Aggregate intrinsic value as of December 31, 2021 was calculated as the difference between the closing price per share of our common stock on the last trading day of 2021, which was $3.92, and the exercise price of the options, multiplied by the number of in-the-money options.

As of December 31, 2021, the unrecognized compensation costs related to outstanding unvested options under our equity incentive plans were $0.3 million. We expect to recognize those costs over a weighted average period of 0.8 years.

We have granted performance stock units to certain executive officers and senior level employees. The number of performance stock units ultimately earned under these awards is calculated based on the Total Shareholder Return (TSR) of our common stock as compared to the TSR of a defined group of peer companies during the applicable three-year performance period. The percentage of performance stock units that vest will depend on our relative position at the end of the performance period and can range from 0% to 200% of the number of units granted.

Based on the performance of our stock relative to our defined group of peer companies for the period 2018 to 2020, PSUs awarded in 2018 vested in 2021 at a rate of 118.6% of the target. The performance adjustment in the table below reflects the impact of the above-target performance.


	Number of Units (in 000s)	Weighted-Average Grant Date Fair Value per Unit
Balance at December 31, 2018	155	$	10.09
PSU granted	401	$	16.90
PSU released	—	—
PSU forfeited	(9)	$	10.09
Balance at December 31, 2019	547	$	15.09
PSU granted	509	$	4.82
PSU released	—	—
PSU forfeited	(94)	$	14.26
Balance at December 31, 2020	962	$	9.74
PSU granted	396	$	9.60
Performance adjustment for 2018 awards	21	$	10.09
PSU released	(133)	$	10.09
PSU forfeited	(36)	$	4.82
Balance at December 31, 2021	1,210	$	10.11

The PSU awards above include 0.3 million of PSUs awarded in 2019. Based on the performance of our stock relative to our defined group of peer companies for the period 2019 to 2021, these awards did not meet the minimum target and the shares returned to the 2011 Equity Plan pool in early 2022.

During 2019, we granted performance stock units to a certain employee. The number of performance stock units that ultimately vest under these awards is dependent on the employee achieving certain discrete operational milestones on or before predetermined measurement dates, the latest of which was December 31, 2021. As of December 31, 2021, there were approximately 29 thousand units of these awards outstanding with a weighted-average grant date fair value of $6.46 per unit. The operational milestones were not met and the awards were cancelled in early 2022.

Our ESPP offers U.S. and some non-U.S. employees the right to purchase shares of our common stock. Our ESPP program has a six-month offering period, with a new period commencing on the first trading day on or after May 31 and November 30 of each year. Employees are eligible to participate through payroll deductions of up to 10% of their compensation. Employees may not purchase more than $25 thousand of stock for any calendar year. Shares are sold to employees under the ESPP for 85% of the lower of the fair market value of a share of our common stock on the first day of the offering period or the last day of the offering period.


	For the Year Ended December 31,
	2021		2020		2019
Restricted stock units, stock options and performance share units	$	15,470	$	13,428	$	10,555
Employee stock purchase plan	631		1,023		838
Total stock-based compensation	$	16,101	$	14,451	$	11,393