The reconciliation of cash, cash equivalents and restricted cash reported in the accompanying unaudited condensed consolidated balance sheets to the total of the same such amounts in the accompanying unaudited condensed consolidated statements of cash flows is as follows (in thousands):


	March 31, 2021		December 31, 2020
Cash and cash equivalents	$	843,336	$	893,219
Restricted cash included in other assets	4,236		4,235
Total cash, cash equivalents and restricted cash	$	847,572	$	897,454


	March 31, 2021		December 31, 2020
Accounts receivable	$	385,204	$	394,037
Allowance for doubtful accounts	(549)		(659)
Product sales rebate and returns reserve	(4,189)		(3,838)
Accounts receivable, net	$	380,466	$	389,540


	March 31, 2021		December 31, 2020
Raw materials	$	234,941	$	219,218
Finished goods	248,227		260,450
Total inventories	$	483,168	$	479,668

Prepaid expenses and other current assets consist of the following (in thousands):


	March 31, 2021		December 31, 2020
Inventory deposit	$	21,478	$	18,783
Prepaid income taxes	3,612		267
Other current assets	69,790		60,556
Other prepaid expenses and deposits	16,715		15,316
Total prepaid expenses and other current assets	$	111,595	$	94,922


	March 31, 2021		December 31, 2020
Equipment and machinery	$	74,401	$	70,655
Computer hardware and software	42,201		40,081
Furniture and fixtures	3,793		3,787
Leasehold improvements	31,499		31,448

Construction-in-process	470		1,441
Property and equipment, gross	152,364		147,412
Less: accumulated depreciation	(119,902)		(115,181)
Property and equipment, net	$	32,462	$	32,231

Depreciation expense was $4.9 million and $5.2 million for the three months ended March 31, 2021 and 2020, respectively.


	March 31, 2021		December 31, 2020
Accrued payroll related costs	$	42,670	$	73,634
Accrued manufacturing costs	43,420		43,181
Accrued product development costs	15,084		6,733
Accrued warranty costs	10,043		9,314
Other	11,249		10,495


Total accrued liabilities	$	122,466	$	143,357

The following table summarizes the activity related to our accrued liability for estimated future warranty costs (in thousands):


	Three Months Ended March 31,
	2021		2020
Warranty accrual, beginning of period	$	9,314	$	6,742
Liabilities accrued for warranties issued during the period	3,456		1,070
Warranty costs incurred during the period	(2,727)		(1,471)
Warranty accrual, end of period	$	10,043	$	6,341

The following table summarizes the beginning and ending balances of our contract assets (in thousands):


	Three Months Ended March 31,
	2021		2020
Contract assets, beginning balance	$	16,380	$	25,565
Contract assets, ending balance	9,762		9,636

A contract liability is recognized when we have received customer payments in advance of our satisfaction of a performance obligation under a cancellable contract. The following table summarizes the activity related to our contract liabilities (in thousands):


	Three Months Ended March 31,
	2021		2020
Contract liabilities, beginning balance	$	85,957	$	61,050
Less: Revenue recognized from beginning balance	(7,913)		(4,847)
Less: Beginning balance reclassified to deferred revenue	(12,492)		(2,433)
Add: Contract liabilities recognized	14,913		10,555
Contract liabilities, ending balance	$	80,465	$	64,325

As of March 31, 2021 and December 31, 2020, $35.9 million and $34.5 million, respectively, of our contract liabilities were recorded within other current liabilities with the remaining balance recorded within other long-term liabilities in the accompanying unaudited condensed consolidated balance sheets.

Deferred revenue is comprised mainly of unearned revenue related to multi-year post-contract support ("PCS") contracts, services and product deferrals related to acceptance clauses. The following table summarizes the activity related to our deferred revenue (in thousands):


	Three Months Ended March 31,
	2021		2020
Deferred revenue, beginning balance	$	650,827	$	575,288
Less: Revenue recognized from beginning balance	(107,442)		(100,377)
Add: Deferral of revenue in current period, excluding amounts recognized during the period	176,646		121,864
Deferred revenue, ending balance	$	720,031	$	596,775

As of March 31, 2021, other performance obligations of $171.7 million were comprised mainly of unbilled multi-year PCS contract amounts that will be recognized as revenue in future periods.

Revenue from total remaining performance obligations represents contract liabilities, deferred revenue and unbilled contract revenue that will be recognized in future periods. As of March 31, 2021, approximately $972.2 million of revenue is

expected to be recognized from total remaining performance obligations. We expect to recognize revenue on approximately 80% of these remaining performance obligations over the next two years and 20% during years 3 to 5.


	Three Months Ended March 31,
	2021		2020
Interest income	$	2,045	$	11,662


Other income (expense), net	(470)		495
Total	$	1,575	$	12,157