	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
	(in thousands)				(in thousands)
Cost of product revenue	$	162	$	81	$	303	$	156
Research and development		631		307		1,769		582
Sales and marketing		913		288		2,309		597
General and administrative		1,140		430		2,727		1,001
Total stock-based compensation	$	2,846	$	1,106	$	7,108	$	2,336

Stock-based compensation of $45,000 and $259,000 was capitalized into inventory for the three and nine months ended September 30, 2020, respectively. Stock-based compensation capitalized into inventory is recognized as cost of product revenue when the related product is sold.

The Company estimates the grant-date fair value of the Company’s option awards using the Black-Scholes option pricing model. During the nine months ended September 30, 2020, the Company granted no new options. The assumptions for the Black-Scholes model for the nine months ended September 30, 2019 were as follows:

On June 17, 2009, the Company adopted the 2009 Equity Incentive Plan (the “2009 Plan”) under which the Board had the authority to issue stock options to employees, directors and consultants.

In February 2019, the Company adopted the 2019 Equity Incentive Plan (the “2019 Plan”), which became effective in connection with the IPO. As a result, effective as of March 6, 2019, the Company may not grant any additional awards under the 2009 Plan. The 2009 Plan will continue to govern outstanding equity awards granted thereunder. The Company initially reserved 2,000,430 shares of common stock for the issuance of a variety of awards under the 2019 Plan, including stock options, stock appreciation rights, awards of restricted stock and awards of restricted stock units. In addition, the number of shares of common stock reserved for issuance under the 2019 Plan will automatically increase on the first day of January commencing on January 1, 2020 and ending on January 1, 2028, in an amount equal to three percent of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by the Company’s Board of Directors. As of September 30, 2020, there were 2,685,328 shares available for issuance under the 2019 Plan.

	Shares Available for Grant		Number of Shares			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Term		Aggregate Intrinsic Value
								(in years)		(in thousands)
Balance, December 31, 2019		1,704,244		3,315,001		$	5.08		7.28	$	128,774
Awards authorized		943,345		—
Options exercised		—		(747,829	)		3.46
Options forfeited		37,739		(37,739	)		4.41
Balance, September 30, 2020		2,685,328		2,529,433		$	5.57		6.96	$	177,632
Vested and exercisable, September 30, 2020				1,620,762		$	4.36		6.56	$	115,785
Vested and expected to vest, September 30, 2020				2,529,433		$	5.57		6.96	$	177,632

Restricted stock units (“RSUs”) are share awards that entitle the holder to receive freely tradable shares of the Company’s common stock upon vesting. The RSUs cannot be transferred and the awards are subject to forfeiture if the holder’s employment terminates prior to the release of the vesting restrictions. The RSUs generally vest over a four-year period with straight-line annual vesting, provided the employee remains continuously employed with the Company. The fair value of the RSUs is equal to the closing price of the Company’s common stock on the grant date.

	Number of Shares			Weighted- Average Grant Date Fair Value Per Share
Balance, December 31, 2019		280,904		$	38.12
RSUs granted		601,183			44.69
RSUs vested		(54,004	)		37.83
RSUs forfeited		(29,375	)		41.36
Balance, September 30, 2020		798,708		$	42.96

In February 2019, the Company adopted the 2019 Employee Stock Purchase Plan (“ESPP”), which became effective as of March 6, 2019. The Company initially reserved 300,650 shares of common stock for purchase under the ESPP. In addition, the number of shares of common stock reserved for issuance under the ESPP will automatically increase on the first day of January commencing on January 1, 2020 and ending January 1, 2028, in an amount equal to one percent of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or such lesser number of shares as determined by the Company’s Board of Directors. At September 30, 2020, a total of 562,486 shares were available for issuance under the ESPP.

Each offering period during which participating employees may purchase stock under the ESPP begins on each September 1 and March 1 and ends on the following February 28 or 29 and August 30, respectively. The first offering period began on September 1, 2019 and ended on February 29, 2020. On each purchase date, which falls on the last date of each offering period, ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the offering date or (2) the fair market value of the common stock on the purchase date. The occurrence and duration of offering periods under the ESPP are subject to the determinations of the Compensation Committee of the Company’s Board of Directors, in its sole discretion.

The fair value of the ESPP shares is estimated using the Black-Scholes option pricing model. The Company recorded $118,000 and $516,000 of stock-based compensation expense related to the ESPP for the three and nine months ended September 30, 2020, respectively. The Company recorded $64,000 of stock-based compensation expense related to the ESPP for the three and nine months ended September 30, 2019, respectively.