We adopted an equity incentive plan in 2009 (“2009 Plan”) that provided for the issuance of incentive and nonqualified common stock options and other share-based awards for employees, directors and consultants. Under the 2009 Plan, the option exercise price for incentive and nonqualified stock options were not to be less than the fair market value of our common stock at the date of grant. Options granted under this plan expire no later than ten years from the grant date and vesting was established at the time of grant. Pursuant to the terms of the 2019 Plan, any shares subject to outstanding options originally granted under the 2009 Plan that terminate, expire or lapse for any reason without the delivery of shares to the holder thereof shall become available for issuance pursuant to awards granted under the 2019 Plan. While no shares are available for future issuance under the 2009 Plan, it continues to govern outstanding equity awards granted thereunder.

The 2019 Plan became effective immediately prior to the closing of our initial public offering in July 2019. The 2019 Plan provides for the issuance of awards in the form of options and other share-based awards for employees, directors and consultants. Under the 2019 Plan, the option exercise price per share shall not be less than the fair market value of a share of stock on the effective date of grant, as defined by the 2019 Plan, unless explicitly qualified under the provisions of Section 409A or Section 424(a) of the Internal Revenue Code of 1986. Additionally, unless otherwise specified, options granted under this plan expire no later than ten years from the grant date and vesting is established at the time of grant. Except for certain option and restricted stock unit grants made to non-employee directors, stock options and restricted stock units granted under the 2019 Plan generally vest over a four-year period, subject to continuous service through each applicable vesting date. As of September 30, 2021, we have authorized 29,118,513 shares of common stock for issuance under the 2019 Plan.

Changes in shares available for grant during the nine months ended September 30, 2021 were as follows:

	Shares Available for Grant
Shares available for grant at December 31, 2020		18,617,001
2019 Equity Incentive Plan reserve increase effective January 1, 2021		6,882,344
Options and restricted stock units granted		(3,295,903	)
Options and restricted stock units forfeited, cancelled or expired		807,623
Shares available for grant at September 30, 2021		23,011,065

Stock option activity under the 2009 Plan and 2019 Plan during the nine months ended September 30, 2021 was as follows:

	Shares Subject to Outstanding Options			Weighted-Average Exercise Price per Share		Aggregate Intrinsic Value (in thousands)
Options outstanding at December 31, 2020		14,433,560		$	12.82	$	668,458
Options granted		2,407,629			41.21
Options forfeited or cancelled		(744,501	)		24.48
Options expired		(21,452	)		18.98
Options exercised		(3,310,804	)		7.04
Options outstanding at September 30, 2021		12,764,432		$	18.98	$	218,168
Options vested and exercisable at September 30, 2021		7,080,906		$	10.29	$	170,535

The weighted-average remaining contractual life for options outstanding as of September 30, 2021 was 7.1 years. The weighted-average remaining contractual life for vested and exercisable options as of September 30, 2021 was 5.9 years.

As of September 30, 2021, $0.1 million was included in the prepaid expenses and other current assets line item on our unaudited condensed consolidated balance sheet for unsettled cash proceeds related to options exercised during the nine months ended September 30, 2021. Of the $23.4 million proceeds from exercise of stock options included on our unaudited condensed consolidated statements of cash flows for the nine months ended September 30, 2021, $0.3 million related to options exercised prior to but settled during the nine months ended September 30, 2021. Of the $15.5 million proceeds from exercise of stock options included on our unaudited condensed consolidated statements of cash flows for the nine months ended September 30, 2020, $0.5 million related to options exercised prior to but settled during the nine months ended September 30, 2020. Furthermore, as of September 30, 2020, there was $0.4 million in unsettled cash proceeds related to options exercised during the nine months ended September 30, 2020.

Restricted stock unit activity under the 2019 Plan during the nine months ended September 30, 2021 was as follows:

	Restricted Stock Units Outstanding			Weighted-Average Grant Date Fair Value per Share
Nonvested outstanding restricted stock units at December 31, 2020		50,000		$	28.10
Restricted stock units granted		888,274			40.21
Restricted stock units forfeited or cancelled		(41,670	)		45.06
Restricted stock units vested		(15,625	)		28.10
Nonvested outstanding restricted stock units at September 30, 2021		880,979		$	39.51

The estimated grant date fair value of options granted during the nine months ended September 30, 2021 and 2020 was estimated using the Black-Scholes option-pricing model with the following assumptions:

	Nine Months Ended September 30,
	2021		2020
Fair value of common stock	$30.86 - $66.50		$17.68 - $48.54
Expected term (in years)	5.27 - 6.08		5.27 - 6.08
Risk-free interest rate	0.5% - 1.1%		0.4% - 1.7%
Expected volatility	67.1% - 70.0%		70.5% - 73.0%
Expected dividend yield		—		—

The weighted-average volatility used in the grant date fair value calculation of options granted during the nine months ended September 30, 2021 and 2020 was 68.6% and 71.2%, respectively.

The determination of the grant date fair value of stock options using a Black-Scholes option-pricing model is affected by the fair value of our common stock, as well as assumptions regarding a number of variables that are complex, subjective and generally require significant judgment to determine. The valuation assumptions were determined as follows:

Fair value of common stock—The fair value of each share of common stock is based on the closing price of our common stock on the date of grant, or other relevant determination date, as reported on The Nasdaq Global Select Market.

Expected term—The expected term of options granted to employees and non-employee directors is determined using the “simplified” method, as illustrated in ASC Topic 718, Compensation—Stock Compensation, as we do not have sufficient exercise history to determine a better estimate of expected term. Under this approach, the expected term is based on the midpoint between the vesting date and the end of the contractual term of the option.

Risk-free interest rate—We utilize a risk-free interest rate in the option valuation model based on U.S. Treasury zero-coupon issues with remaining terms similar to the expected terms of the options.

Expected volatility—As we do not have sufficient trading history for our common stock, the expected volatility is based on the historical volatility of our publicly traded industry peers utilizing a period of time consistent with our estimate of expected term.

Expected dividend yield—We do not anticipate paying any cash dividends in the foreseeable future and, therefore, use an expected dividend yield of zero in the option valuation model.

The grant date fair value of restricted stock units granted is based on the closing price of our common stock on the date of grant, or other relevant determination date, as reported on The Nasdaq Global Select Market.

The compensation costs related to stock options and restricted stock units for the three and nine months ended September 30, 2021 and 2020, respectively, are included on our unaudited condensed consolidated statements of operations as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Cost of revenue	$	650	$	206	$	1,386	$	587
Research and development		3,637		2,216		10,311		5,912
Sales and marketing		3,369		1,759		9,166		4,583
General and administrative		3,987		2,289		10,513		6,436
Total share-based compensation expense	$	11,643	$	6,470	$	31,376	$	17,518

As of September 30, 2021, unrecognized share-based compensation expense related to unvested stock options was $97.6 million, which is expected to be recognized over a remaining weighted-average period of 2.9 years. Additionally, as of September 30, 2021, unrecognized share-based compensation expense related to unvested restricted stock units was $30.3 million, which is expected to be recognized over a remaining weighted-average period of 3.4 years.