3. Property and Equipment

Property and equipment are recorded at cost and depreciated over the estimated useful lives of the underlying assets (three to five years) using the straight-line method. As of June 30, 2019 and December 31, 2018, property and equipment consists of (in thousands):

Total depreciation and amortization expense was $22,000 and $45,000 for the three and six months ended June 30, 2019, which includes amortization expense of $18,000 and $35,000 related to finance lease right-of-use lab equipment, respectively. Total depreciation and amortization expense were $14,000 and $29,000 for the three and six months ended June 30, 2018, respectively, and included no amortization expense for finance lease right-of-use assets. For additional finance leases information, refer to Note 12 – Commitments and Contingencies.