12. Stock Warrants

The following is a description of the common and convertible preferred stock warrant activity of the Company:


(in thousands, except per share amounts)	Warrants for the Purchase of Common Stock			Weighted Average Exercise Price		Warrants for the Purchase of Convertible Preferred Stock			Weighted Average Exercise Price
Balance—December 31, 2010		2,937		$	2.93		306		$	3.48
Expired		(1	)	$	2.47		—			—
Exercised		(296	)	$	2.46		(4	)	$	1.89

Balance—December 31, 2011		2,640		$	2.98		302		$	3.50
Conversion		302		$	3.50		(302	)	$	3.50
Exercised		(100	)	$	2.63		—			—

Balance—December 31, 2012		2,842		$	3.05		—			—
Exercised		(65	)	$	2.82

Balance—December 31, 2013		2,777		$	3.05		—			—

During the year ended December 31, 2012, warrants to purchase approximately 100,000 shares of common stock were cashless exercised and 71,000 shares of common stock were issued. During the year ended December 31, 2013, warrants to purchase approximately 65,000 shares of common stock were cashless exercised and 35,000 shares of common stock were issued.