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. We previously entered into a $2.1 million letter of credit with one of our financial institutions in connection with one of our leases. As of March 31, 2021, we were not party to any finance leases. Our leases have remaining terms of 1.1 years to 12.4 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 March 31, 2021, 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 March 31, 2021 was as follows:

The following table reconciles our undiscounted operating lease cash flows to our operating lease liabilities as of March 31, 2021 (in thousands):

2021 (excluding the three months ended March 31, 2021)	$	6,120
2022		13,554
2023		13,318
2024		13,030
2025		13,419
Thereafter		89,236
Total undiscounted lease payments		148,677
Less:
Imputed interest rate		(34,873	)
Tenant improvement receivables		(14,244	)
Total operating lease liabilities	$	99,560
Less: current portion		(4,308	)
Operating lease liabilities, less current portion	$	95,252

Operating lease expense was $3.1 million and $1.1 million for the three months ended March 31, 2021 and 2020, respectively. Variable lease expense for operating leases was $0.7 million and $0.5 million for the three months ended March 31, 2021 and 2020, respectively.

Cash paid for amounts included in the measurement of lease liabilities was $0.5 million, net of $1.2 million of cash received for tenant improvement allowances during the three months ended March 31, 2021. Cash paid for amounts included in the measurement of lease liabilities was $0.8 million, net of $0.3 million of cash received for tenant improvement allowances during the three months ended March 31, 2020.

In March 2021, we entered into a lease to rent approximately 27,000 square feet of a warehouse in Bothell, Washington. Rent obligations commence six months after lease commencement 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. This lease will be assessed for classification and a lease liability and corresponding ROU asset will be recorded upon lease commencement. Future non-cancellable undiscounted lease payments, exclusive of operating and maintenance costs, total $7.0 million. Furthermore, in connection with this lease, the landlord agreed to fund $1.2 million in improvements.