3. Cash Equivalents and Marketable Securities

The following tables summarize the estimated fair value of our cash equivalents and marketable securities and the gross unrealized gains and losses (in thousands):

	March 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Cash equivalents:
Money market funds	$	4,800	$	—	$	—	$	4,800
Reverse repurchase agreements		16,250		—		—		16,250
Total cash equivalents		21,050		—		—		21,050
Marketable securities:
US government and agency securities		4,967		3		—		4,970
Total marketable securities	$	4,967	$	3	$	—	$	4,970

	December 31, 2015
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Cash equivalents:
Money market funds	$	3,988	$	—	$	—		$	3,988
Reverse repurchase agreements		16,250		—		—			16,250
Total cash equivalents		20,238		—		—			20,238
Marketable securities:
US government and agency securities		3,769		—		(1	)		3,768
Total marketable securities	$	3,769	$	—	$	(1	)	$	3,768

All of the investments held as of March 31, 2016 and December 31, 2015 had maturities of less than one year.

As of March 31, 2016 and December 31, 2015, the Company held zero and five debt securities, respectively, that were in an unrealized loss position for less than one year. The aggregate fair value of debt securities in an unrealized loss position at December 31, 2015 was $2.5 million with no individual securities in a significant unrealized loss position. The Company evaluated its securities for other-than-temporary impairment and considered the decline in market value for the securities to be primarily attributable to current economic and market conditions. It is not more likely than not that the Company will be required to sell the securities before recovery of the amortized cost basis, and the Company does not intend to do so prior to the recovery of the amortized cost basis. Based on this analysis, these marketable securities were not considered to be other-than-temporarily impaired as of December 31, 2015.