The Company's investments are classified as available-for-sale. The following table summarizes the amortized cost, gross unrealized gains and losses, and fair value of available-for-sale investments as of December 31, 2021:


	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Aggregate Fair Value
United States government debt securities	$	121,036	$	—	$	(331)	$	120,705
Certificates of deposit	97		—		—		97
Total	$	121,133	$	—	$	(331)	$	120,802

The following table summarizes the amortized cost, gross unrealized gains and losses, and fair value of available-for-sale investments as of December 31, 2020:


	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Aggregate Fair Value
United States government debt securities	$	48,048	$	14	$	(1)	$	48,061
Certificates of deposit	264		—		—		264
Total	$	48,312	$	14	$	(1)	$	48,325

See Notes 2 and 7 for further discussion on the Company's method for determining the fair value of its assets.

The estimated fair value of available-for-sale investments classified by their contractual maturities as of December 31, 2021 was:


Due within one year	$	72,240
After one year through two years	48,562
Total	$	120,802

Changes in market interest rates and bond yields cause certain investments to fall below their cost basis, resulting in unrealized losses on investments. The unrealized losses of the Company's debt security investments are not significant as of December 31, 2021.