	December 31, 2020
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
U.S. treasury bills and government agency obligations	$	20,110	$	166	$	—	$	20,276
U.S. corporate debt		11,505		42		—		11,547
Foreign corporate debt		15,508		49		—		15,557
U.S. commercial paper		13,997		—		—		13,997
Foreign commercial paper		41,976		—		—		41,976
Total	$	103,096	$	257	$	—	$	103,353

	December 31, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
U.S. treasury bills and government agency obligations	$	140,567	$	59	$	—	$	140,626
U.S. corporate debt securities		80,159		11		—		80,170
Foreign corporate debt securities		23,188		15		—		23,203
U.S. commercial paper		32,801		—		—		32,801
Foreign commercial paper		42,274		—		—		42,274
Total	$	318,989	$	85	$	—	$	319,074

The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. We regularly monitor and evaluate the realizable value of our marketable securities. We did not recognize any impairment losses for the years ended December 31, 2020 and 2019.

Unrealized gains and losses associated with our investments are reported in accumulated other comprehensive loss. For both years ended December 31, 2020 and 2019, we recorded $172,000 in net unrealized gains associated with our short-term investments. For the year ended December 31, 2018, we recorded $77,000 in net unrealized losses associated with our short-term investments.

Realized gains and losses associated with our investments, if any, are reported in the statements of operations and comprehensive loss. We recognized $8,000 in realized gains during the year ended December 31, 2019. We did not recognize any realized gains or losses during the years ended December 31, 2020 and 2018.

	December 31,
	2020		2019
Raw materials	$	18,994	$	6,635
Work in process		6,847		12,571
Finished goods		16,064		5,762
Total inventory	$	41,905	$	24,968

As of December 31, 2020, total inventory included $37.8 million related to CINVANTI and $4.1 million related to SUSTOL. As of December 31, 2019, total inventory included $23.5 million related to CINVANTI and $1.5 million related to SUSTOL. In addition, cost of product sales for the years ended December 31, 2020 and 2019 included charges of $0.1 million and $3.3 million, respectively, resulting from the write-off of short-dated SUSTOL inventory.

	December 31,
	2020			2019
Scientific equipment	$	29,135		$	24,603
Leasehold improvements		878			206
Computer equipment and software		1,461			1,314
Furniture, fixtures and office equipment		2,135			1,520
Property and equipment, gross		33,609			27,643
Less: accumulated depreciation and amortization		(10,872	)		(8,025	)
Property and equipment, net	$	22,737		$	19,618

Depreciation and amortization expense for the years ended December 31, 2020, 2019 and 2018 was $2.8 million, $2.0 million and $1.5 million, respectively. As of December 31, 2020 and 2019, $14.1 million and $13.5 million of property and equipment, respectively, was in process and not depreciated during the respective years.

Accrued payroll and employee liabilities consist of the following (in thousands):

	December 31,
	2020		2019
Accrued employee salaries and benefits	$	1,691	$	3,047
Accrued bonuses		8,479		9,545
Accrued vacation		3,427		2,656
Total accrued payroll and employee liabilities	$	13,597	$	15,248

	December 31,
	2020		2019
Accrued product sales allowances	$	24,571	$	27,939
Accrued consulting and professional fees		3,450		7,742
Accrued accounts payable		104		310
Other accrued liabilities		244		544
Total other accrued liabilities	$	28,369	$	36,535