A summary of the Company’s stock option activity and related information is as follows:

				Options Outstanding
	Shares Available for Grant			Number of Stock Options Outstanding			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
											(In thousands)
Balance at December 31, 2019		948,415			11,165,846		$	7.44		7.6	$	1,907
Additional options authorized		9,137,667
Options granted		(736,579	)		736,579			1.97
Options exercised		—			(20,579	)		0.72
Options cancelled		2,165,317			(2,165,317	)		6.27
Withheld shares to pay for taxes on released RSUs		45,913
RSUs granted		(5,962,979	)
RSUs forfeited		557,428
Balance at June 30, 2020		6,155,182			9,716,529		$	7.26		7.9	$	907
Vested and exercisable at June 30, 2020					4,870,115		$	7.04		7.3	$	626
Vested and expected to vest at June 30, 2020					9,297,531		$	7.28		7.8	$	871

The weighted-average grant date fair value of options granted to employees was $1.20 and $4.79 per share during the six months ended June 30, 2020 and 2019, respectively. The grant date fair values of options vested was $5.7 million and $3.3 million during the six months ended June 30, 2020 and 2019, respectively.

Aggregate intrinsic value represents the difference between the estimated fair value of the underlying common stock and the exercise price of outstanding, in-the-money options. The aggregate intrinsic values of options exercised was $0.0 million and $6.5 million during the six months ended June 30, 2020 and 2019, respectively.

At June 30, 2020, total unrecognized compensation cost related to stock options granted to employees, net of estimated forfeitures, was $18.9 million which is expected to be recognized over a weighted-average period of 2.2 years.

The determination of the fair value of stock options on the date of grant using an option-pricing model is affected by the estimated fair value of the Company’s common stock, as well as assumptions regarding a number of complex and subjective variables. The variables used to calculate the fair value of stock options using the Black-Scholes option-pricing model include actual and projected employee stock option exercise behaviors, expected price volatility of the Company’s common stock, the risk-free interest rate and expected dividends. Each of these inputs is subjective and generally requires significant judgment to determine.

The fair value of employee stock options is estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:

From time to time, the Company grants Restricted Stock Units, or RSUs, to its board of directors and certain employees for their services. Certain board members elected RSUs in lieu of retainer and committee service fees, which vest over a period of time from the grant date and will be released and settled upon termination of the board member’s services or the occurrence of a change in control event. In January and June 2020, the Company granted RSUs to its Board of Directors as part of the director compensation program. The January 2020 RSUs granted to board members vested in full on the date of our 2020 Annual Meeting of Stockholders. The June 2020 RSUs granted to board members vest in full on the date of our 2021 Annual Meeting of Stockholders. In January and June 2020, the Company granted RSUs to the Company’s CEO that vest annually at 42% on the first anniversary date of the grant, and 29% on the second and third anniversary date of the grant. In January and March of 2020, the Company granted RSUs to several employees. These RSUs vest in equal annual installments over three years from the grant date and are subject to the participants continuing service to the Company over that period. The fair value of RSUs is based on the closing market price of the Company’s common stock on the grant date.

	RSUs
	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2019		4,379,777		$	5.14
RSUs granted		5,962,979			2.78
RSUs vested		(393,772	)		6.20
RSUs forfeited		(557,428	)		3.16
Unvested at June 30, 2020		9,391,556		$	3.72
Vested and unreleased		142,336
Outstanding at June 30, 2020		9,533,892

The total grant date fair value of RSUs awarded was $16.4 million for the six months ended June 30, 2020. The total fair value of RSUs vested was $2.4 million for the six months ended June 30, 2020. The total grant date fair value of RSUs awarded was $5.3 million for the six months ended June 30, 2019. The total fair value of RSUs vested was $0.2 million for the six months ended June 30, 2019.

As of June 30, 2020, total unrecognized stock-based compensation cost related to RSUs was $24.5 million, which is expected to be recognized over a weighted-average period of 2.1 years. At June 30, 2020, 8,765,862 shares of RSUs are expected to vest.

Total stock-based compensation expense recognized for employee stock awards and employee stock purchase plan shares in the Company’s condensed consolidated statements of operations and comprehensive loss is classified as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Cost of revenue	$	270	$	628	$	508	$	1,210
Research and development		636		946		1,156		1,824
Selling and marketing		343		365		559		642
General and administrative		4,554		2,968		9,081		5,783
Total stock-based compensation expense	$	5,803	$	4,907	$	11,304	$	9,459

Stock-based compensation relating to stock-based awards granted to consultants was insignificant during the three and six months ended June 30, 2020 and 2019.