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Stock Options
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Stock Options Stock Options
Stock option plans
As of June 30, 2017, there were 3,141,625 options outstanding under the Company’s 2007 Stock Incentive Plan. The 2007 Plan expired in June 2017. Any cancellations or forfeitures of options granted under the 2007 Stock Incentive Plan will increase the options available under the 2017 Stock Incentive Plan as described below.
In June 2017 the Company’s shareholders approved the 2017 Stock Incentive Plan (the 2017 Plan or the Plan). Under the 2017 Plan initially, up to 2,255,000 shares of common stock may be granted to the Company's employees, officers, directors, consultants and advisors in the form of options, RSUs or other stock-based awards. The number of shares of common stock issuable under the Plan cumulatively increases annually by 4% of the outstanding shares or such lesser amount determined by the Board. The terms of the awards made under the Plan are determined by the Board, subject to the provisions of the Plan. In January 2021, the number of shares of common stock issuable under the 2017 Plan was increased by 2,753,651 shares. As of December 31, 2021 there were 1,308,183 shares available for future issuance under the Plan. During the year ended December 31, 2021, the Company granted to employees 715,716 RSUs and options to purchase 2,870,720 shares of common stock under the 2017 Plan.
Under the 2017 Plan, both with respect to incentive stock options and nonqualified stock options, the exercise price per share will equal the fair market value of the common stock on the date of grant, as determined by the Board, and the vesting period is generally four years. Options granted under the Plan expire no later than 10 years from the date of grant. Options under the 2007 Plan were granted at an exercise price established by the Board (or a committee thereof) that was not less than the fair market value of the underlying common stock on the date of grant and subject to such vesting provisions determined by the Board (or a committee thereof). The Board may accelerate vesting or otherwise adjust the terms of granted options in the case of a merger, consolidation, dissolution, or liquidation of the Company.
Inducement awards
The Company grants to its employees, upon approval by the Board, options to purchase shares of common stock as an inducement to employment in accordance with Nasdaq Listing Rule 5635(c)(4). The securities are issued pursuant to Section 4(a)(2) under the Securities Act of 1933, as amended, relating to transactions by an issuer not involving any public offering. These options are subject to terms substantially the same as the options granted under the 2017 Plan. As of December 31, 2021 there were options to purchase 757,500 shares of common stock granted as inducement awards outstanding.
Stock option activity
A summary of the activity is as follows:
Number
of Shares
Weighted-
Average
Exercise Price
Weighted Average
Remaining
Contractual Term
Aggregate
Intrinsic Value
Outstanding at January 1, 20216,112,948 $7.84 7.3$114,729 
Granted3,308,220 $16.78 
Exercised(421,381)$4.36 
Cancelled(657,358)$11.76 
Outstanding at December 31, 20218,342,429 $11.25 7.2$8,458 
Vested and expected to vest at December 31, 20218,342,429 $11.25 7.2$8,458 
Exercisable at December 31, 20213,997,529 $7.63 5.5$7,539 
The weighted-average grant date fair value of options granted during the years ended December 31, 2021, 2020 and 2019, was $11.71, $7.99 and $2.47 per share, respectively. The total intrinsic value of options exercised during the years ended December 31, 2021, 2020 and 2019, was $4,299, $11,147, and $202, respectively. The aggregate intrinsic value represents the difference between the exercise price and the selling price received by option holders upon the exercise of stock options during the period.
Cash received from the exercise of stock options was $1,837, $3,138 and $175 for the years ended December 31, 2021, 2020 and 2019, respectively.
Restricted stock units
The Company periodically issues RSUs with a service condition to certain officers and other employees that typically vest between one year and four years from the grant date.

A summary of the RSU activity under the 2017 Plan is as follow:
Number
of Shares		Weighted-Average
Remaining
Contractual Term		Aggregate
Intrinsic Value		Weighted-Average
Grant Date
Fair Value
Unvested at January 1, 2021716,767 1.0$19,073 $6.00 
Granted715,716 — $18.39 
Vested(419,336)— $5.33 
Forfeited(195,538)— $12.34 
Unvested at December 31, 2021817,609 1.5$5,086 $15.68 
The total fair value of RSUs vested during the years ended December 31, 2021, 2020 and 2019, was $5,790, $0, and, $0, respectively.
Stock-based compensation expense
The Company uses the provisions of ASC 718, Stock Compensation, to account for all stock-based awards to employees and non-employees.
The measurement date for awards is generally the date of grant. Stock-based compensation expense is recognized over the requisite service period, which is generally the vesting period, using the straight-line method.
The following table presents stock-based compensation expense by award type included within the Company’s consolidated statement of operations and comprehensive loss:
Year ended December 31,
202120202019
Stock options
$14,528 $5,725 $4,230 
Restricted stock units
3,522 1,187 410 
Employee stock purchase plan
359 260 232 
Stock-based compensation expense included in Total operating expenses
$18,409 $7,172 $4,872 
The following table presents stock-based compensation expense as reflected in the Company’s consolidated statements of operations and comprehensive loss:
Year ended December 31,
202120202019
Research and development$9,984 $3,841 $2,245 
General and administrative8,425 3,331 2,627 
Stock-based compensation expense included in Total operating expenses$18,409 $7,172 $4,872 
As of December 31, 2021, there was $38,958 and $9,929 of unrecognized compensation expense related to unvested stock options and unvested RSUs, respectively, that is expected to be recognized over a weighted average period of 2.6 years and 2.8 years, respectively.
The fair value of each option award is estimated on the date of grant using the Black–Scholes option pricing model with the following weighted average assumptions:
December 31,
202120202019
Risk-free interest rate
0.9 %1.2 %2.3 %
Expected dividend yield
— %— %— %
Expected term (years)
6.066.055.99
Expected stock price volatility
82 %74 %74 %
Expected volatility for the Company’s common stock is determined based on the historical volatility of comparable publicly traded companies. The risk-free interest rate is based on the yield of U.S. Treasury securities consistent with the expected term of the option. No dividend yield was assumed as the Company has not historically and does not expect to pay dividends on its common stock. The expected term of the options granted is based on the use of the simplified method, in which the expected term is presumed to be the mid-point between the vesting date and the end of the contractual term.
The fair value of RSUs is determined based on the closing price of the Company’s common stock on the date of grant.
Employee Stock Purchase Plan
During the year ended December 31, 2017, the Board adopted and the Company's stockholders approved the 2017 employee stock purchase plan (the 2017 ESPP). The Company initially reserved 225,000 shares of common stock for issuance under the 2017 ESPP, plus an annual increase, to be added as of January 1st of each year, equal to the least of (i) 450,000 shares of common stock; (ii) one percent of the number of shares of common stock outstanding as of the close of business on the immediately preceding December 31st; and (iii) the number of shares of common stock determined by the Board on or prior to such date for such year, up to maximum of 4,725,000 shares of common stock in the aggregate. During the years ended December 31, 2021 and 2020 the Company issued 78,253 and 80,267 shares, respectively, under the 2017 ESPP. As of December 31, 2021, there were 566,565 shares available for issuance.