4. 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 (in thousands):

	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

	December 31, 2016
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
Commercial paper	$	160,729	$	6	$	(153	)	$	160,582
Corporate notes		74,430		3		(64	)		74,369
Total securities	$	235,159	$	9	$	(217	)	$	234,951

Realized gains or losses from the sale of investments 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 was determined based on the specific identification method. The amount of realized gains and realized losses on investments for the periods presented have not been material.

The amortized costs and estimated fair values of the Company’s available-for-sale securities by contractual maturities as of December 31, 2017 were as follows (in thousands):

	Amortized Cost		Fair Value
Amounts maturing within one year	$	204,739	$	204,385
Amounts after one year through five years		22,140		22,082
Total investment securities	$	226,879	$	226,467

Inventories (in thousands)

	December 31,
	2017		2016
Raw materials	$	51,602	$	44,862
Finished goods		46,517		40,359
Total inventories	$	98,119	$	85,221

Property and Equipment, Net (in thousands)

	December 31,
	2017			2016
Laboratory equipment	$	2,416		$	1,567
Computer equipment and software		5,076			2,388
Furniture and fixtures		2,241			2,051
Leasehold improvements		1,221			1,214
Construction in process		2,734			2,274
Total		13,688			9,494
Less: Accumulated depreciation and amortization		(4,869	)		(2,362	)
Property and equipment, net	$	8,819		$	7,132

Depreciation and amortization expense for the years ended December 31, 2017, 2016 and 2015 was $2.5 million, $1.7 million and $0.6 million, respectively.

Accrued Liabilities (in thousands)

	December 31,
	2017		2016
Accrued payroll and related expenses	$	26,108	$	17,732
Accrued professional fees		4,734		1,067
Accrued taxes		2,827		2,110
Accrued clinical and research expenses		1,279		1,545
Accrued interest		243		243
Accrued warranty		708		645
Accrued other		3,491		2,686
Total accrued liabilities	$	39,390	$	26,028