A summary of the Company's cash, cash equivalents and marketable securities is shown below:

		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value

		(in thousands)
September 30, 2011
Cash and cash equivalents:
	Cash and money market funds	$	348,199	$	—	$	—		$	348,199
	U.S. Treasury securities		7,788		—		—			7,788
	Government-sponsored enterprise securities		186,586		1		(29	)		186,558

Total cash and cash equivalents		$	542,573	$	1	$	(29	)	$	542,545

Marketable securities:
	Government-sponsored enterprise securities (due within 1 year)	$	116,144	$	6	$	(10	)	$	116,140

Total marketable securities		$	116,144	$	6	$	(10	)	$	116,140

Total cash, cash equivalents and marketable securities		$	658,717	$	7	$	(39	)	$	658,685

December 31, 2010
Cash and cash equivalents:
	Cash and money market funds	$	193,845	$	—	$	—		$	193,845
	U.S. Treasury securities		4,770		—		—			4,770
	Government-sponsored enterprise securities		44,587		1		(6	)		44,582

Total cash and cash equivalents		$	243,202	$	1	$	(6	)	$	243,197

Marketable securities:
	U.S. Treasury securities (due within 1 year)	$	103,230	$	1	$	(11	)	$	103,220
	Government-sponsored enterprise securities (due within 1 year)		684,969		87		(62	)		684,994

Total marketable securities		$	788,199	$	88	$	(73	)	$	788,214

Total cash, cash equivalents and marketable securities		$	1,031,401	$	89	$	(79	)	$	1,031,411

Alios' $50.6 million of cash and money market funds as of September 30, 2011, recorded on the Company's condensed consolidated balance sheet in "Restricted cash and cash equivalents (Alios)," are not included in the above table.

In the three months ended September 30, 2011 and 2010, the Company had proceeds of $135.1 million and $461.7 million, respectively, from sales and maturities of available-for-sale securities. In the nine months ended September 30, 2011 and 2010, the Company had proceeds of $923.1 million and $979.8 million, respectively, from sales and maturities of available-for-sale securities.

Realized gains and losses are determined using the specific identification method and are included in interest income on the Company's condensed consolidated statements of operations. There were no gross realized gains and losses for the three and nine months ended September 30, 2011 and 2010.