4. Investments

Marketable Investments

Held-to-Maturity Investments

Marketable investments classified as held-to-maturity consist of the following (in thousands):

As of December 31, 2015	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Government-sponsored enterprises	$	53,254	$	3	$	(119	)	$	53,138
Corporate notes and bonds		106,722		37		(67	)		106,692

Total	$	159,976	$	40	$	(186	)	$	159,830


Reported under the following captions on the consolidated balance sheet:
Current marketable investments	$	121,974
Noncurrent marketable investments		38,002

	$	159,976

As of December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Government-sponsored enterprises	$	127,212	$	118	$	(39	)	$	127,291
Corporate notes and bonds		293,288		260		(108	)		293,440

Total	$	420,500	$	378	$	(147	)	$	420,731


Reported under the following captions on the consolidated balance sheet:
Current marketable investments	$	297,842
Noncurrent marketable investments		122,658

	$	420,500

The following table summarizes gross unrealized losses and the length of time marketable investments have been in a continuous unrealized loss position (in thousands):

	As of December 31,
	2015					2014
	Fair Value		Gross Unrealized Loss			Fair Value		Gross Unrealized Loss
Government-sponsored enterprises:
Continuous unrealized loss position less than one year	$	48,138	$	(119	)	$	15,293	$	(39	)
Continuous unrealized loss position greater than one year		—		—			—		—

		48,138		(119	)		15,293		(39	)
Corporate notes and bonds:
Continuous unrealized loss position less than one year		63,840		(67	)		86,824		(97	)
Continuous unrealized loss position greater than one year		—		—			3,443		(11	)

		63,840		(67	)		90,267		(108	)

Total	$	111,978	$	(186	)	$	105,560	$	(147	)

We attribute the unrealized losses on held-to-maturity securities as of December 31, 2015 and 2014, to the variability in related market interest rates. We do not intend to sell these securities, nor is it more likely than not that we will be required to sell them prior to the end of their contractual terms. Furthermore, we do not believe that these securities expose us to undue market risk or counterparty credit risk. As such, we do not consider these securities to be other than temporarily impaired.

The following table summarizes the contractual maturities of held-to-maturity marketable investments (in thousands):

	As of December 31, 2015
	Amortized Cost		Fair Value
Due in less than one year	$	121,974	$	121,932
Due in one to two years		38,002		37,898
Due in three to five years		—		—
Due after five years		—		—

Total	$	159,976	$	159,830

Investments Held at Cost

As of December 31, 2015, we maintain in the aggregate, non-controlling equity investments of approximately $137.2 million in privately-held corporations, including a $100.0 million investment in the preferred stock of Synthetic Genomics Inc. (SGI), which we purchased in two separate $50.0 million transactions in May 2014 and September 2015. We account for these investments under the cost method since we do not have the ability to exercise significant influence over these companies and their fair values are not readily determinable. The fair value of these investments has not been estimated at December 31, 2015, as we have not identified any events or developments indicating that their carrying amounts may be impaired. We include these investments within other assets on our accompanying consolidated balance sheets.

In addition to the SGI investments noted above, we entered into a separate multi-year research and development collaboration agreement with SGI in May 2014, whereby SGI will develop engineered primary pig cells with modified genomes for use in our xenotransplantation program. This collaboration was initially focused primarily on lungs and was expanded in September 2015 to include an additional focus on kidneys. Under this agreement, each party assumes its own research and development costs and SGI may receive royalties and milestone payments from development and commercialization of organs.