9. OTHER BALANCE SHEET COMPONENTS

Property and equipment

Property and equipment consist of the following (in thousands):

​

Depreciation and amortization expense was $0.2 million, $0.9 million and $1.2 million for the years ended December 31, 2023, 2022 and 2021, respectively. Certain property and equipment were either sold, retired or disposed of, which related costs and accumulated depreciation were removed from the balance sheet, and any resulting gain or loss were reflected in statements of operations in the period realized.

Intangible asset

Intangible asset consist of the following (in thousands):

​

Intangible asset pertain to amortized cost of capitalized milestone payment to Forma, incurred upon and after regulatory approval of an acquired product. See “Note 4 - Sponsored Research and License Agreements and Government Contracts” for related discussions. Such cost is being amortized on a straight-line basis over the estimated useful life of approximately 14 years.

Amortization expense recorded within cost of sales in the statements of operations were $1.1 million and $0.1 million for the years ended December 31, 2023 and 2022, respectively.

The following table presents the estimated future amortization expense of intangible asset (in thousands):

​

Other accrued liabilities

Other accrued liabilities consist of the following (in thousands):

​