Leases are recorded in accordance with FASB Topic 842, which the Company adopted on January 1, 2019 under the modified retrospective transition method. The Company determines if an arrangement includes a lease at the inception of the agreement. For each of the Company’s lease arrangements, a right-of-use asset representing its right to use an underlying asset for the lease term and a lease liability representing its obligation to make lease payments are recorded. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the net present value of lease payments over the term of the lease. See “Item 2—Properties” in this Annual Report for a description of the significant operating leases of offices and laboratory space the Company had as of December 31, 2020. The Company did not assume lease extension options would be exercised in its right-of-use asset and lease liability amounts. The Company has three additional operating leases that are included in its lease accounting but are not considered significant.

The Company determined that the identified operating leases did not contain non-lease components and required no further allocation of the total lease cost. Additionally, the agreements in place did not contain information to determine the rate implicit in the leases. As such, the Company calculated the incremental borrowing rate based on the assumed remaining lease term for each lease in order to calculate the present value of the remaining lease payments. At December 31, 2020, the weighted average incremental borrowing rate and the weighted average remaining lease term for the operating leases held by the Company were 5.25% and 12.6 years, respectively.

In July 2020, the Company entered into an amendment to its existing lease at 852 Winter Street, Waltham, Massachusetts (as amended, the “852 Winter Street Lease”). The amendment became effective on October 7, 2020. The 852 Winter Street Lease governs approximately 180,000 square feet of corporate office space, administrative areas and laboratories. The amendment served to, among other things, extend the term of the 852 Winter Street Lease for a period of approximately five years, to commence in March 2021 and expire in April 2026 with respect to approximately 163,000 square feet of the 852 Winter Street Lease premises (the “Base Premises”) and to commence in September 2021 and expire in October 2026 with respect to approximately 17,000 square feet of the 852 Winter Street Lease premises (the “Additional Premises”). The Company expects to make annual lease payments of approximately $5.7 million for the Base Premises and $0.5 million for the Additional Premises, subject to annual increases. Under the terms of the 852 Winter Street Lease, the Company will have the option to extend the term for an additional five-year period. The Company determined that the amendment did not grant an additional right-of-use and therefore was not deemed to be a separate new lease and that the 852 Winter Street Lease should be reassessed and remeasured as of the effective date of the amendment. The Company accounted for the amendment prospectively.

In March 2018, the Company entered into a lease agreement for approximately 220,000 square feet of office and laboratory space located in a building that was to be built at 900 Winter Street, Waltham, Massachusetts (the “900 Winter Street Lease”). The initial term of the lease commenced on January 20, 2020 (the “Commencement Date”) and includes an option to extend the term for an additional ten-year period. As the lease had not commenced as of December 31, 2019 and the Company: (a) did not have the right to obtain or control the leased premises during the construction period; (b) did not have the right of payment for the partially constructed assets, and thus, the partially constructed assets could have potentially been leased to another tenant; and (c) did not legally own or control the land on which the property improvements were being constructed, the lease assets were not included as right-of-use assets at December 31, 2019. The future lease payments outlined below do not include the 900 Winter Street Lease payments as of December 31, 2019 under Topic 842.

As of December 31, 2020 and 2019, right-of-use assets arising from operating leases were $131.7 million and $12.4 million, respectively, and right-of-use liabilities arising from operating leases were $135.2 million and $13.8 million, respectively. During the year ended December 31, 2020 and 2019, cash paid for amounts included for the measurement of lease liabilities was $17.3 million and $9.1 million, respectively. The Company recorded operating lease expense of $16.3 million, $8.1 million and $10.8 million for the years ended December 31, 2020, 2019 and 2018, respectively.

Future lease payments under non-cancelable leases as of December 31, 2020 consisted of the following:

	December 31,
(In thousands)	2020
2021	$	16,882
2022		17,001
2023		17,266
2024		17,536
2025		17,810
Thereafter		109,311
Total lease payments	$	195,806
Less: imputed interest		(60,610	)
Total operating lease liabilities	$	135,196

For comparable purposes, future lease payments under non-cancelable leases as of December 31, 2019 consisted of the following:

	December 31,
(In thousands)	2019
2020	$	9,053
2021		2,727
2022		500
2023		509
2024		520
Thereafter		2,579
Total lease payments	$	15,888
Less: imputed interest		(2,080	)
Total operating lease liabilities	$	13,808