We have operating lease agreements for laboratory and office facilities in Seattle, Washington, South San Francisco, California and New York City, New York, as well as server space. Additionally, 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 nine months ended September 30, 2021. Rent obligations commenced in October 2021 and the lease expires 120 months thereafter, subject to an early termination option after the seventh year and an option to twice extend the lease for five years. Furthermore, the landlord agreed to fund $1.2 million in improvements in connection with this lease. For the nine months ended September 30, 2021, ROU assets obtained in exchange for operating lease liabilities was $5.4 million.

As of September 30, 2021, we were not party to any finance leases. Our leases have remaining terms of 0.6 years to 11.9 years and include options to extend certain of the leases for up to 10.0 years and terminate certain of the leases after as few as 3.0 years. We adjust lease terms for these options only when it is reasonably certain we will exercise these options. As of September 30, 2021, it was reasonably certain that we would exercise our option to terminate two of our leases after 3.0 years.

We previously entered into a $2.1 million letter of credit with one of our financial institutions in connection with one of our leases.

Other information related to our operating leases as of September 30, 2021 was as follows:

The following table reconciles our undiscounted operating lease cash flows to our operating lease liabilities as of September 30, 2021 (in thousands):

2021 (excluding the nine months ended September 30, 2021)	$	2,757
2022		14,184
2023		13,964
2024		13,692
2025		14,098
Thereafter		93,518
Total undiscounted lease payments		152,213
Less:
Imputed interest rate		(33,755	)
Tenant improvement receivables		(5,306	)
Total operating lease liabilities	$	113,152
Less: current portion		(5,108	)
Operating lease liabilities, less current portion	$	108,044

Operating lease expense was $3.1 million and $1.4 million for the three months ended September 30, 2021 and 2020, respectively, and $9.3 million and $3.7 million for the nine months ended September 30, 2021 and 2020, respectively. Variable lease expense for operating leases was $0.8 million and $0.7 million for the three months ended September 30, 2021 and 2020, respectively, and $2.3 million and $1.8 million for the nine months ended September 30, 2021 and 2020, respectively.

Cash paid for amounts included in the measurement of lease liabilities was $5.6 million and cash received for tenant improvement allowances was $11.5 million during the nine months ended September 30, 2021. Cash paid during the nine months ended September 30, 2020 for amounts included in the measurement of lease liabilities was $1.8 million, net of $1.8 million of cash received for tenant improvement allowances.