10. Stockholders’ Equity

We recorded stock-based 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,
	2016		2015		2016		2015
Cost of revenue	$	1,049	$	991	$	2,825	$	2,835
Research and development		18,662		22,308		63,078		70,485
Sales and marketing		1,541		2,045		5,738		5,181
General and administrative		3,223		5,092		9,341		21,302
Total stock-based expense	$	24,475	$	30,436	$	80,982	$	99,803

The following table shows stock option activity for the nine months ended September 30, 2016 (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, 2015		23,215		$	1.93	$	35,949		5.36
Granted		15,000			2.54
Forfeited and cancelled		(2,857	)		3.01
Exercised		(3,693	)		0.27
Balance as of September 30, 2016		31,665		$	2.32	$	34,718		6.58

The following table shows a summary of ZSU activity for the nine months ended September 30, 2016 (in thousands, except weighted-average grant date fair value):

	Outstanding ZSUs
				Weighted-		Aggregate
				Average Grant Date		Intrinsic Value of
	Shares			Fair Value		Unvested ZSUs
Unvested as of December 31, 2015		62,436		$	3.06	$	167,328
Granted		31,596			2.27
Vested		(22,386	)		2.82
Forfeited and cancelled		(9,289	)		2.86
Unvested as of September 30, 2016		62,357		$	2.78	$	181,459

The following table shows a summary of changes in accumulated other comprehensive income by component for the three and nine months ended September 30, 2016 (in thousands):

	Foreign Currency Translation			Unrealized Gains (Losses) on Available-for-Sale Securities			Total
Balance as of June 30, 2016	$	(100,521	)	$	3		$	(100,518	)
Other comprehensive income (loss) before reclassifications		(6,030	)		(7	)		(6,037	)
Amounts reclassified from accumulated other comprehensive income (loss)		—			—			—
Net current-period other comprehensive income (loss)		(6,030	)		(7	)		(6,037	)
Balance as of September 30, 2016	$	(106,551	)	$	(4	)	$	(106,555	)

	Foreign Currency Translation			Unrealized Gains (Losses) on Available-for-Sale Securities			Total
Balance as of December 31, 2015	$	(52,261	)	$	(127	)	$	(52,388	)
Other comprehensive income (loss) before reclassifications		(54,290	)		124			(54,166	)
Amounts reclassified from accumulated other comprehensive income (loss)		—			(1	)		(1	)
Net current-period other comprehensive income (loss)		(54,290	)		123			(54,167	)
Balance as of September 30, 2016	$	(106,551	)	$	(4	)	$	(106,555	)

In October 2015, we announced that our board of directors authorized a stock repurchase program allowing us to repurchase up to $200 million of our outstanding shares of Class A common stock (the “Q4 2015 Repurchase Program”). From January 1, 2016 to February 2, 2016 we repurchased 42.2 million shares under the Q4 2015 Repurchase Program at an average price per share of $2.40, for a total of $101.9 million. All of our stock repurchases under the Q4 2015 Repurchase Program were made through open market purchases under Rule 10b5-1 plans.

The Q4 2015 Repurchase Program was completed in February 2016 – in the aggregate we repurchased 80.2 million shares under the Q4 2015 Repurchase Program at an average price of $2.50 for a total of $200 million.