We have one active stockholder-approved stock-based compensation plan, the 2018 Long-Term Incentive Plan (the “2018 Plan”). In June 2018, the 2018 Plan replaced our former 2009 Incentive Award Plan (the “2009 Plan”). Under the 2018 Plan, we may grant restricted stock awards and units, incentive and nonqualified stock options, performance unit awards, stock appreciation rights, and other stock-based awards to employees, consultants, and members of our Board of Directors. Stock-based awards generally vest one-third on the first anniversary of the date of grant, and in equal annual installments thereafter over the remaining two years vesting period. Stock options must generally be exercised, if at all, no later than 10 years from the date of grant. In the event of a change in control, all award types with the exception of performance unit awards, will vest in full effective immediately prior to the consummation of the change in control. For performance unit awards, if a change in control occurs prior to the end date and the participant remains employed prior to the change in control, the shares vest based on the achievement of the performance goals as of the date of which the change in control occurs.

The stated maximum availability of common stock under the 2018 Plan is 18 million shares, except for additional availability provided on a one-for-one basis for awards formerly issued under the 2009 Plan that are terminated, forfeited, cancelled or expire unexercised. Awards issued under the 2018 Plan reduce share availability on a one-to-one basis for stock options and on a 1.5-to-one basis for restricted stock awards and restricted stock units. Accordingly, as of December 31, 2021, 4.4 million awards were available for grant under the 2018 Plan, assuming all were issued in the form of stock options, but would be reduced to 3.0 million awards available for grant if all were issued in the form of restricted stock.

It is our policy that before stock is issued through the exercise of stock options, we must first receive all required cash payment for such shares (whether through an upfront cash exercise or net-settlement exercise). At the time of vesting of restricted stock, by our policy, requisite shares are automatically sold on the open market by our designated broker to the extent required to cover the employee’s federal and state taxes due.

Stock-based awards are governed by agreements between us and the recipients. Incentive stock options and nonqualified stock options may be granted under the 2018 Plan at an exercise price of not less than 100% of the fair market value of our common stock on the respective date of grant and for certain recipients may not be less than 110% of such fair market value. The grant date is generally the date the terms of the award are approved by the Compensation Committee of our Board of Directors.

Under the terms of our 2009 Employee Stock Purchase Plan (the “ESPP”), eligible employees can purchase common stock through scheduled payroll deductions. The purchase price is equal to the closing price of our common stock on the first or last day of the offering period (whichever is less), minus a 15% discount. We use the Black-Scholes option-pricing model, in combination with the discounted employee price, in determining the value of ESPP expense to be recognized during each offering period. A participant may purchase a maximum of 50,000 shares of common stock during a six-month offering period, not to exceed $25,000 at full market value on the offering date during each plan year.

As of December 31, 2021, a total of 8.2 million shares of common stock are authorized and remain available for issuance under the ESPP. Beginning on January 1, 2010, and each January 1st thereafter, the number of shares of common stock available for issuance under the ESPP shall automatically increase by an amount equal to the lesser of (i) one million shares or (ii) an amount determined by the ESPP administrator. However, in no event shall the number of shares of common stock available for future sale under the ESPP exceed 10 million shares, subject to capitalization adjustments occurring due to dividends, splits, dissolution, liquidation, mergers, or changes in control.

Employee stock-based compensation expense for the years ended December 31, 2021 and 2020 was recognized (reduced for estimated forfeitures) on a straight-line basis over the vesting period. Forfeitures are estimated at the time of grant and are prospectively revised if actual forfeitures differ from those estimates. We estimate forfeitures of stock options using the historical exercise behavior of our employees. For purposes of this estimate, we have applied an estimated forfeiture rate of 11% and 15% for the years ended December 31, 2021 and 2020, respectively.


	Year Ended December 31,
	2021	2020
Expected option life (in years)(a)	5.57	5.46
Risk-free interest rate(b)	0.56% - 1.32%	0.34% - 1.61%
Volatility(c)	80.0% - 82.7%	74.5% - 81.4%
Dividend yield(d)	0%	0%
Weighted-average grant-date fair value per stock option	$1.78	$1.63

Stock option activity during the years ended December 31, 2021 and 2020 was as follows:


	Number of Shares	Weighted- Average Exercise Price/Share		Weighted- Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value










Outstanding — December 31, 2019	6,439,936	$	9.61
Granted	2,032,000	2.34
Exercised	(3,542)	3.70			$	708	(1)
Forfeited	(170,187)	8.29
Expired	(641,584)	8.55
Outstanding — December 31, 2020	7,656,623	$	7.80
Granted	2,397,684	2.66
Exercised	(1,250)	3.04			$	1.5	(1)
Forfeited	(34,565)	10.05
Expired	(513,031)	9.29
Outstanding — December 31, 2021	9,505,461	$	6.42	5.86	$	—	(2)
Vested (exercisable) — December 31, 2021	6,371,266	$	8.04	4.26	$	—	(2)
Unvested (unexercisable) — December 31, 2021	3,134,195	$	3.13	9.10	$	—	(2)

(2) Represents the total difference between our closing stock price on the last trading day of 2021 and the stock option exercise price, multiplied by the number of in-the-money options as of December 31, 2021. The amount of intrinsic value will change based on the fair market value of our stock.

The following table summarizes information with respect to stock option grants as of December 31, 2021:


	Outstanding				Exercisable
Exercise Price	Granted Stock Options Outstanding	Weighted- Average Remaining Contractual Life (Years)	Weighted- Average Exercise Price		Granted Stock Options Exercisable	Weighted- Average Exercise Price
$1.29 - 4.96	4,397,491	7.6	$	2.52	1,507,886	$	2.58
$4.97 - 6.91	1,854,119	4.33	6.00		1,854,119	6.00
$6.92 - 9.00	1,376,034	3.41	7.80		1,300,617	7.77
$9.01 - 12.00	925,709	4.17	11.17		769,707	11.17
$12.01 - 22.64	952,108	5.99	18.61		938,937	18.63
	9,505,461	5.86	$	6.42	6,371,266	$	8.04

As of December 31, 2021 , there was unrecognized compensation expense of $4.2 million related to unvested stock options, which we expect to recognize over a weighted average period of 2.1 years. For the year ended December 31, 2021 , we recorded stock option expense of $4.5 million related to issued stock options.

A summary of restricted stock award activity is as follows:


	Number of Restricted Stock Awards	Weighted Average Fair Value per Share at Grant Date








Unvested — December 31, 2019	1,659,759	$	11.67
Granted	4,026,518	2.68
Vested	(753,475)	11.23
Forfeited	(436,757)	6.03
Unvested — December 31, 2020	4,496,045	4.29
Granted	2,820,259	3.33
Vested	(2,272,064)	5.06
Forfeited	(608,233)	3.85
Unvested — December 31, 2021	4,436,007	$	3.33

For the years ended December 31, 2021 and 2020, we recorded stock-based compensation expense on our issued restricted share awards of $9.6 million and $9.4 million, respectively. As of December 31, 2021, there was approximately $9.7 million of unrecorded expense that will be recognized over an estimated weighted average period of 1.9 years. These unvested shares are included in our reported issued and outstanding common stock as of December 31, 2021.

Our outstanding restricted stock units substantially relate to awards that contain “market-based” vesting conditions that are issued to our executive officers. These conditions are specified in each award agreement and result in a variable number of shares that become issuable at the assessment date, after review and approval by our Compensation Committee. A summary of restricted stock unit activity is as follows:


	Number of Restricted Stock Units	Weighted Average Fair Value per Share at Grant Date





Outstanding — December 31, 2019	385,919	$	18.00
Granted	6,800	2.36
Market-based achievement adjustment at vesting	(128,334)	—
Share issuance	(861)	10.69

Outstanding — December 31, 2020	263,524	26.39
Granted	2,125	3.61
Market-based achievement adjustment at vesting	75,000	—
Share issuance	(151,386)	28.09
Forfeited	(4,751)	4.03
Outstanding — December 31, 2021	184,512	$	23.53

For the years ended December 31, 2021 and 2020, we recorded stock-based compensation expense on our issued restricted stock units of $1.1 million and $1.1 million, respectively. As of December 31, 2021, there was an immaterial amount of unrecorded expense to be recognized.

During the years ended December 31, 2021 and 2020, we granted 2.1 million and 1.7 million stock appreciation rights (“SARs”), respectively, to our Named Executive Officers. On the date of grant, the fair value of these SARs were estimated using the Black-Scholes option-pricing model and 25% immediately vested. There were no forfeitures made during the year. We recognized stock-based compensation expense of $4.2 million and $1.5 million, respectively, within our Consolidated Statements of Operations for the years ended December 31, 2021 and 2020. As of December 31, 2021, there was approximately $1.9 million of unrecorded expense that will be recognized over an estimated weighted average period of 2.0 years.

Beginning in March 2020, employee 401(k) matching contributions were made with cash rather than stock on a prospective basis. As a result of this change, the shares issued to participants 401(k) accounts were substantially lower in the current year period compared to prior year periods.