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Stockholders' Equity | 3 Months Ended
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Mar. 31, 2021
Equity [Abstract] |
Stockholders' Equity | 10. Stockholders’ Equity Public Offering Warrants In June 2014, as a component of our public offering, we sold 600,000 pre-funded warrants to purchase shares of our common stock. The pre-funded warrants have an exercise price of $0.01 per share and expire on June 30, 2021. As of March 31, 2021, 195,574 warrants from the June 2014 public offering remain outstanding. Option Plan Activity The following table summarizes the stock option activity for the three months ended March 31, 2021:
|  | Shares (in thousands) |  |  | Weighted- average Exercise Price |  |  | Weighted- average Remaining Contractual Term (Years) |
Outstanding at December 31, 2020 |  |  | 18,912 |  |  | $ | 19.59 |  |  |  | 7.21 |
Granted |  |  | 102 |  |  | $ | 18.96 |  |  |  |  |
Exercised |  |  | (71 | ) |  | $ | 15.86 |  |  |  |  |
Expired and forfeited |  |  | (148 | ) |  | $ | 20.81 |  |  |  |  |
Outstanding at March 31, 2021 |  |  | 18,795 |  |  | $ | 19.59 |  |  |  | 6.92 |
The following table summarizes the restricted stock unit activity for the three months ended March 31, 2021:
|  | Shares (in thousands) |  |  | Weighted Average Grant Date Fair Value |
Outstanding at December 31, 2020 |  |  | 603 |  |  | $ | 15.84 |
Granted |  |  | 17 |  |  | $ | 18.93 |
Released |  |  | (39 | ) |  | $ | 15.98 |
Expired and forfeited |  |  | (11 | ) |  | $ | 15.85 |
Outstanding at March 31, 2021 |  |  | 570 |  |  | $ | 15.93 |
For the three months ended March 31, 2021, 70,885 shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of $1.1 million. Stock-based Compensation The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):
|  | Three Months Ended March 31, |
|  | 2021 |  |  | 2020 |
Research and development |  | $ | 4,937 |  |  | $ | 5,287 |
General and administrative |  |  | 2,786 |  |  |  | 3,431 |
Sales and marketing |  |  | 3,763 |  |  |  | 3,256 |
Total stock-based compensation expense |  | $ | 11,486 |  |  | $ | 11,974 |
As of March 31, 2021, there was $104.4 million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 2.7 years. The fair value of RSUs is estimated based on the closing market price of our common stock on the date of the grant. RSUs generally vest quarterly over a four-year period. We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:
|  | March 31, |
|  | 2021 |  |  | 2020 |
Risk-free interest rate |  |  | 0.7 | % |  |  | 1.3 | %
Dividend yield |  |  | 0.0 | % |  |  | 0.0 | %
Volatility |  |  | 69.4 | % |  |  | 66.2 | %
Expected life (years) |  |  | 6 |  |  |  | 6 |
We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan, as amended, at the beginning of each new offering period using the Black-Scholes option pricing model. There were no new offering periods for the three months ended March 31, 2021 and 2020.

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- Definition The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(e)(1)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -URI http://asc.fasb.org/topic&trid=2208762 + Details
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