10. Stockholders’ Equity

We recorded stock-based compensation expense related to grants of employee and consultant stock options, restricted stock, and restricted stock units (“ZSUs”) in our consolidated statements of operations as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Cost of revenue	$	435	$	1,049	$	1,425	$	2,825
Research and development		10,097		18,662		32,293		63,078
Sales and marketing		1,891		1,541		5,429		5,738
General and administrative		3,166		3,223		10,199		9,341
Total stock-based compensation expense	$	15,589	$	24,475	$	49,346	$	80,982

The following table shows stock option activity for the nine months ended September 30, 2017 (in thousands, except weighted-average exercise price and weighted-average contractual term):

	Outstanding Options
				Weighted-		Aggregate		Weighted-
				Average		Intrinsic Value of		Average
				Exercise		Stock Options		Contractual Term
	Stock Options			Price		Outstanding		(in years)
Balance as of December 31, 2016		36,858		$	2.08	$	26,411		6.81
Granted		848			3.62
Forfeited and cancelled		(3,014	)		3.60
Exercised		(2,166	)		1.29
Balance as of September 30, 2017		32,526		$	2.03	$	57,771		6.44

The following table shows a summary of ZSU activity for the nine months ended September 30, 2017 (in thousands, except weighted-average grant date fair value):

	Outstanding ZSUs
				Weighted-
				Average Grant Date		Aggregate
				Fair Value		Intrinsic Value of
	Shares			(per share)		Unvested ZSUs
Unvested as of December 31, 2016		59,452		$	2.66	$	152,792
Granted		16,262			3.36
Vested		(15,941	)		2.84
Forfeited and cancelled		(13,862	)		2.63
Unvested as of September 30, 2017		45,911		$	2.86	$	173,544

The following table shows a summary of changes in accumulated other comprehensive income (loss) by component for the nine months ended September 30, 2017 (in thousands):

	Foreign Currency Translation			Unrealized Gains (Losses) on Available-for-Sale Securities			Total
Balance as of December 31, 2016	$	(128,671	)	$	(23	)	$	(128,694	)
Other comprehensive income (loss) before reclassifications		37,479			(8	)		37,471
Less: Amounts reclassified from accumulated other comprehensive income (loss)		—			—			—
Net current-period other comprehensive income (loss)		37,479			(8	)		37,471
Balance as of September 30, 2017	$	(91,192	)	$	(31	)	$	(91,223	)

In November 2016, we announced that our Board of Directors authorized a share repurchase program allowing us to repurchase up to $200 million of our outstanding shares of Class A common stock (“2016 Share Repurchase Program”). In the first and second quarter of 2017, we repurchased 34.2 million shares for our Class A common stock under the repurchase program at a weighted average price of $2.70 per share for a total of $92.9 million. In the third quarter of 2017, we did not repurchase any shares under the repurchase program.

All shares repurchased under the 2016 Share Repurchase Program were made through open market purchases under 10b5-1 plans, and were subsequently retired.