5. Balance Sheet Components

Investments

The fair value of the Company’s cash equivalents and short-term investments approximates their respective carrying amounts due to their short-term maturity. The following is a summary of the gross unrealized gains and unrealized losses on the Company’s investment securities, excluding investments in money market funds (in thousands):

	March 31, 2018
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
Commercial paper (i)	$	70,036	$	7	$	(84	)	$	69,959
Corporate notes		166,682		—		(686	)		165,996
Total securities	$	236,718	$	7	$	(770	)	$	235,955

	December 31, 2017
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
Commercial paper	$	61,167	$	—	$	(81	)	$	61,086
Corporate notes		165,712		1		(332	)		165,381
Total securities	$	226,879	$	1	$	(413	)	$	226,467

i)	Includes $4.0M of commercial paper that is classified as cash and cash equivalents on the consolidated balance sheet.

Realized gains or losses and other-than-temporary impairments, if any, on available-for-sale securities are reported in other income (expense), net as incurred. The cost of securities sold is determined based on the specific identification method. The amount of realized gains and realized losses on investments recorded for the periods presented has not been material.

The contractual maturities of the Company’s investment securities as of March 31, 2018 were as follows (in thousands):

	Amortized Cost		Fair Value
Amounts maturing within one year	$	236,718	$	235,955
Amounts maturing after one year through five years		—		—
Total investment securities	$	236,718	$	235,955

Inventories (in thousands)

	March 31,		December 31,
	2018		2017
Raw materials	$	45,693	$	51,602
Finished goods		51,804		46,517
Total inventories	$	97,497	$	98,119

Property and Equipment, Net (in thousands)

	March 31,			December 31,
	2018			2017
Laboratory equipment	$	2,535		$	2,416
Computer equipment and software		6,804			5,076
Furniture and fixtures		2,241			2,241
Leasehold improvements		1,221			1,221
Construction in process		5,656			2,734
Total		18,457			13,688
Less: Accumulated depreciation and amortization		(5,684	)		(4,869	)
Property and equipment, net	$	12,773		$	8,819

The Company recognized depreciation and amortization expense on property and equipment as follows (in thousands):

	Three Months Ended
	March 31,
	2018		2017
Depreciation and amortization expense	$	815	$	485

Accrued Liabilities (in thousands)

	March 31,		December 31,
	2018		2017
Accrued payroll and related expenses	$	17,609	$	26,108
Accrued professional fees		4,635		4,734
Accrued taxes		2,794		2,827
Accrued clinical and research expenses		876		1,279
Accrued interest		998		243
Accrued warranty		820		708
Accrued other		3,407		3,491
Total accrued liabilities	$	31,139	$	39,390