We have operating lease agreements for the laboratory and office facilities that we occupy in various locations, as well as server space.

In July 2011, we entered into a non-cancelable lease agreement with an, at the time, minority shareholder for our current headquarters 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 August 2019, we entered into an agreement to rent approximately 100,000 square feet in a to-be-constructed 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. Due to our significant involvement during the construction process of the leased building, we qualified as the deemed owner of the building under build-to-suit lease accounting guidance that proceeded ASC 842. The resulting asset and long-term financing obligation recorded on our balance sheet as of December 31, 2019 for the cost of the building was derecognized upon adoption of ASC 842. The lease commenced in December 2020, when the landlord delivered the premises to us for construction of certain tenant improvements. Cash payment for rent begins in October 2021 and the lease term ends in August 2033, subject to our option to twice extend the lease for five years. We plan to occupy the new building in 2021, once interior construction is finished. 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, 2020, we have incurred $5.2 million in certain tenant improvement costs, of which $0.7 million has been reimbursed by the landlord. The remaining $4.5 million is presented as a reduction in the cash flows used to measure our ROU asset and lease liabilities on our balance sheet as of December 31, 2020 and will be reimbursed by the landlord. The lease also requires us to pay additional amounts for operating and maintenance expenses.

In October 2019, we entered into an agreement to lease approximately 14,750 square feet in a separate Seattle, Washington location, pursuant to a lease expiring in October 2029, which is subject to our ability to exercise an early termination right after the third year. In connection with the lease, the landlord has agreed to provide a tenant improvement allowance in the maximum amount of $0.7 million. The lease also requires us to pay additional amounts for operating 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 cost 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 December 2019, we entered into an agreement to lease approximately 3,100 square feet of office space in New York City, New York, pursuant to a lease that expires November 2025, subject to our ability to exercise an early termination right in 2023.

As of December 31, 2020, we were not party to any finance leases. Our leases have remaining terms of 1.3 years to 12.7 years and include options to extend certain of the leases up to 10.0 years and terminate certain of the leases after 3.0 years. We adjust lease terms for these options only when it is reasonably certain we will exercise these options. As of December 31, 2020, it was reasonably certain that we would exercise our option to terminate two of our leases after 3.0 years.

Other information related to our operating leases as of December 31, 2020 was as follows:

The following table reconciles our undiscounted operating lease cash flows to our operating lease liabilities as of December 31, 2020 (in thousands):

2021	$	7,828
2022		13,554
2023		13,318
2024		13,030
2025		13,419
Thereafter		88,463
Total undiscounted lease payments		149,612
Less:
Imputed interest rate		(35,848	)
Tenant improvement receivables		(5,902	)
Total operating lease liabilities	$	107,862
Less: current portion		(3,529	)
Operating lease liabilities, less current portion	$	104,333

Operating lease expense was $5.5 million for the year ended December 31, 2020. Variable lease expense for operating leases was $2.6 million for the year ended December 31, 2020. Rent expense recognized under ASC 840, inclusive of operating and maintenance costs, was $5.3 million and $4.1 million for the year ended December 31, 2019 and 2018, respectively.

Cash paid for amounts included in the measurement of lease liabilities for the year ended December 31, 2020 was $3.2 million, net of $2.5 million of cash received for tenant improvement allowances. For the year ended December 31, 2020, ROU assets obtained in exchange for operating lease liabilities was $109.1 million, inclusive of the $33.0 million recognized from the adoption of ASC 842.