The following table summarizes the amortized cost and estimated fair value of the Company's U.S. Treasury securities and U.S. government-sponsored enterprise securities, which are considered to be available-for-sale investments and were included in short-term investments as of September 30, 2023 and in short-term investments and long-term investments as of December 31, 2022 (in thousands):


	September 30, 2023
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury securities	$	32,957	$	3	$	(47	)	$	32,913
U.S. government-sponsored enterprise securities		79,576		—		(258	)		79,318
	$	112,533	$	3	$	(305	)	$	112,231


	December 31, 2022
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
U.S. Treasury securities	$	32,174	$	3	$	(34	)	$	32,143
U.S. government-sponsored enterprise securities		66,106		68		(85	)		66,089
Long-term investments:
U.S. Treasury securities		7,242		—		(4	)		7,238
U.S. government-sponsored enterprise securities		34,686		—		(43	)		34,643
	$	140,208	$	71	$	(166	)	$	140,113

Certain short-term debt securities with original maturities of less than 90 days are included in cash and cash equivalents on the condensed consolidated balance sheets and are not included in the tables above. As of September 30, 2023 and December 31, 2022, all short-term investments had contractual maturities within one year. As of December 31, 2022, all long-term investments had contractual maturities between one to two years.

The aggregate fair value of available-for-sale securities held by the Company in an unrealized loss position for less than 12 months as of September 30, 2023 was $86.6 million. The unrealized losses on the Company’s investments of $0.3 million and $0.2 million as of September 30, 2023 and December 31, 2022, respectively, were caused by interest rate increases which resulted in the decrease in market value of these securities. There were no available-for-sale securities in a continuous unrealized loss position for greater than 12 months. Because the decline in fair value is attributable to changes in interest rates and not credit quality, and because the Company does not intend to sell the investments and it is not more likely than not that the Company will be required to sell the investments before recovery of their amortized cost bases, which may be maturity, there are no allowances for credit losses as of September 30, 2023.