We have operating lease agreements for laboratory, office and warehouse facilities that we occupy in various locations.

In July 2011, we entered into a non-cancelable lease agreement with an, at the time, minority shareholder for office and laboratory space in Seattle, Washington. The lease terms were subsequently amended multiple times, most recently in August 2019, when we expanded the existing premises to approximately 65,500 square feet. Cash payment for rent of the expanded premises commenced January 2020, four months after the landlord delivered the expanded premises to us for construction of certain tenant improvements, and the lease term for both the existing premises and the expanded premises ends October 2032, subject to our option to twice extend the lease for five years. The amended lease also requires us to pay additional amounts for operating and maintenance expenses.

In October 2023, we vacated this leased space. As such, we assessed the right-of-use asset and related leasehold improvements (together, the "asset group") for impairment by first comparing the carrying amount of the asset group to future net undiscounted cash flows projected to be generated over the remaining lease term. These projections include management's estimates of cash inflows from potential sublease income and outflows for operating and maintenance expenses. The carrying amount was found to be unrecoverable, thus we assessed the asset group's fair value. The extent to which the asset group's carrying amount exceeds its fair value represents the impairment cost to be recognized. Fair value was determined using the income approach, whereby we discounted estimated net cash flows using a rate commensurate with our estimated incremental borrowing rate. As a result of this assessment, which included unrecoverable operating and maintenance costs, we determined that the asset group was to be fully impaired. As such, an impairment charge of $25.4 million was recognized during the year ended December 31, 2023, $21.2 million related to the right-of-use asset and $4.2 million related to the long-lived leasehold improvements, all of which were held for use.

In August 2019, we entered into an agreement to rent approximately 100,000 square feet in what was a to-be-constructed, new headquarters building in Seattle, Washington. In connection with the lease, we entered into a $2.1 million letter of credit with one of our existing financial institutions. The lease commenced in December 2020, when the landlord delivered the premises to us for construction of certain tenant improvements. We occupied the new building in 2021, cash payment for rent began in October 2021 and the lease term ends in August 2033, subject to our option to twice extend the lease for five years. In connection with this lease, the landlord agreed to fund $20.0 million in improvements, which was subsequently reduced to $14.8 million as a result of our change requests made during landlord construction of the building, net of an administration fee. As of December 31, 2022, we incurred $14.9 million in certain tenant improvement costs, all of which had been reimbursed by the landlord. As of December 31, 2021, we incurred $14.9 million in certain tenant improvement costs, of which $10.9 million had been reimbursed by the landlord. The remaining $4.0 million is presented as a reduction in the cash flows used to measure our ROU asset and lease liabilities on the consolidated balance sheet as of December 31, 2021. The lease also requires us to pay additional amounts for operating and maintenance expenses.

In April 2018, we entered into a lease agreement to lease approximately 13,400 square feet in South San Francisco, California. The lease term is through March 2026 and provides for one five-year extension option. We are responsible for our share of allocable operating expenses, tax expenses and utilities costs during the duration of the lease term. In connection with the lease, the landlord funded agreed-upon improvements. The landlord was solely responsible for the $2.4 million cost of such improvements. The lease agreement was amended in February 2020 to lease approximately 19,900 additional square feet and provides for a $0.6 million tenant improvement allowance.

In March 2021, we executed a lease to rent approximately 27,000 square feet of a warehouse in Bothell, Washington, which we classified as an operating lease upon commencement during the year ended December 31, 2021. Rent obligations commenced in October 2021 and the lease expires October 2031, subject to an early termination option after the seventh year and an option to twice extend the lease for five years. The lease requires us to pay certain operating expenses. Furthermore, the landlord agreed to fund $1.2 million in improvements in connection with this lease.

As of December 31, 2023, we were not party to any finance leases. Our leases have remaining terms ranging from 2.2 years to 9.7 years and include options to extend certain of the leases up to 10.0 years and terminate certain of the leases after 7.0 years of leasing. We adjust lease terms for these options only when it is reasonably certain we will exercise these options.

Other information related to our operating leases as of December 31, 2023 and 2022 was as follows:


	December 31,
	2023			2022
Weighted-average remaining lease term (in years)		8.91			9.75
Weighted-average discount rate		4.6	%		4.6	%

The following table reconciles our undiscounted operating lease cash flows to our operating lease liabilities, less current portion balance on the consolidated balance sheet as of December 31, 2023 (in thousands):


2024	$	13,692
2025		14,098
2026		12,330
2027		11,944
2028		12,282
Thereafter		56,962
Total undiscounted lease payments		121,308
Less: Imputed interest rate		(22,536	)
Total operating lease liabilities		98,772
Less: Current portion		(9,384	)
Operating lease liabilities, less current portion	$	89,388

Operating lease expense was $11.4 million, $12.4 million and $12.4 million for the year ended December 31, 2023, 2022 and 2021, respectively. Variable lease expense for operating leases was $7.1 million, $7.2 million and $3.5 million for the year ended December 31, 2023, 2022 and 2021, respectively.

Cash paid for amounts included in the measurement of lease liabilities for the year ended December 31, 2023 was $13.4 million. Cash paid for amounts included in the measurement of lease liabilities for the year ended December 31, 2022 was $9.2 million, net of $5.2 million of cash received for tenant improvement allowances. Cash paid for amounts included in the measurement of lease liabilities was $8.3 million and cash received for tenant improvement allowances was $11.5 million during the year ended December 31, 2021. For the year ended December 31, 2021, ROU assets obtained in exchange for operating lease liabilities was $5.4 million.