Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 73,808	$ 50,312
Marketable securities	497,841	479,470
Equity securities	2,253
Accounts receivable	6,825	21,574
Income tax receivable	904	502
Prepaid expenses and other current assets	6,476	6,547
Total current assets	588,107	558,405
Property and equipment, net	16,799	15,805
Patents, licenses, and other intangible assets, net	14,637	14,421
Marketable securities - long term	38,232	71,526
Income tax receivable		402
Other assets	9,188	9,691
Total assets	666,963	670,250
Current liabilities
Accounts payable	10,313	10,189
Accrued expenses	7,292	8,995
Lease liabilities	2,136	2,169
Deferred revenue	46,176	45,205
Total current liabilities	65,917	66,558
Lease liabilities, net of current portion	8,041	8,565
Deferred revenue, net of current portion		1,926
Total liabilities	73,958	77,049
Commitments and contingencies
Stockholders' equity
Preferred stock, $0.01 par value: 10,000,000 authorized shares; -0- issued and outstanding shares at March 31, 2020 and December 31, 2019
Common stock, $0.01 par value: 200,000,000 authorized shares at March 31, 2020 and December 31, 2019; 57,001,253 issued and outstanding at March 31, 2020 and 56,902,301 issued and outstanding at December 31, 2019	570	569
Additional paid-in capital	895,855	887,873
Accumulated other comprehensive income (loss)	1,056	1,161
Accumulated deficit	(304,476)	(296,402)
Total stockholders' equity	593,005	593,201
Total liabilities and stockholders' equity	$ 666,963	$ 670,250