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Balance Sheet Components - Prepaid and Other Current Assets (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Prepaid and Other Current Assets    
Interest receivable $ 174 $ 329
Prepaid research and development manufacturing expenses 205 240
Prepaid facility expenses 160 157
Prepaid insurance 85 150
Other 512 486
Total of prepaid and other current assets $ 1,136 $ 1,362