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Balance Sheet Components - Prepaid and Other Current Assets (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Prepaid and Other Current Assets    
Interest receivable $ 399 $ 337
Prepaid research and development manufacturing expenses 250 75
Prepaid facility expenses 138 149
Prepaid insurance 77 166
Other 333 265
Total of prepaid and other current assets $ 1,197 $ 992