In June 2015, the Company entered into a lease agreement for the lease of research facility space in Cambridge, Massachusetts, with a commencement date of November 15, 2015, or the 2015 Lease. The lease was subsequently amended in both 2017 and 2020 and now expires in September 2022 with no further option to extend. The 2015 Lease contains escalating rent clauses, which require higher rent payments in future years. With the adoption of ASC 842 in January 2019, the Company recorded a right-of-use asset and corresponding lease liability.

In May 2016, the Company entered into a sublease agreement for its primary office and research facility in Cambridge, Massachusetts, with a commencement date of December 23, 2016, or the 2016 Sublease. The sublease expires in December 2026, and the Company has an option to extend the term of the sublease for an additional five-year period if, at the time of expiration of the initial term, the sublessor does not intend to utilize the space for itself or its affiliates. The 2016 Sublease contains escalating rent clauses, which require higher rent payments in future years. With the adoption of ASC 842 in January 2019, the Company recorded a right-of-use asset and corresponding lease liability. The right-of-use asset and corresponding lease liability does not include the additional five-year period under the renewal option as the Company is not reasonably certain to exercise that option.

In May 2019, the Company entered into a lease agreement for office facility space in Cambridge, Massachusetts, with a commencement date of June 1, 2019, or the 2019 Lease. The lease expires in November 2026, and the Company has an option to extend the term of the lease for an additional five-year period based on certain conditions within the Company’s control. The 2019 Lease contains escalating rent clauses which require higher rent payments in future years. At lease commencement, the Company recorded a right-of-use asset and corresponding lease liability. The right-of-use asset and corresponding lease liability does not include the additional five-year period under the renewal option as the Company is not reasonably certain to exercise that option.

In December 2019, Casebia became a wholly-owned subsidiary of the Company. In connection therewith, Casebia assigned its sublease for an office and research facility in Cambridge, Massachusetts, or the 2019 Sublease, to the Company. The sublease expires in March 2024 and the Company has an option to extend the term of the sublease for an additional five-year period if, at the time of expiration of the initial term, the sublessor does not intend to utilize the space for itself or its affiliates. The 2019 Sublease contains escalating rent clauses which require higher rent payments in future years. The Company recorded a right-of-use asset and corresponding lease liability. The right-of-use asset and corresponding lease liability does not include the additional five-year period under the renewal option as the Company is not reasonably certain to exercise that option.

In May 2020, the Company entered into a lease agreement for a cell therapy manufacturing facility in Framingham, Massachusetts, or the Framingham Lease, for clinical and commercial production of the Company’s investigational cell therapy product candidates. The lease expires in March 2036 and the Company has an option to extend the term of the lease for two additional seven-year periods. The lease commenced in the fourth quarter of 2020, and at lease commencement, the Company recorded a right-of-use asset and corresponding lease liability. The right-of-use asset and corresponding lease liability does not include the additional seven-year periods under the renewal option as the Company is not reasonably certain to exercise that option.

In July 2020, the Company entered into a lease agreement for an office and laboratory facility in Boston, Massachusetts, or the 2020 Boston Lease. The 2020 Boston Lease is expected to commence in the first half of 2022. In connection therewith, the Company has committed to making at least $292.5 million in rental payments over a lease term of 152 months. The Company is also obligated to make certain future payments related to the construction of leasehold improvements.

In addition, the Company rents certain office space in Zug, Switzerland, on a short-term basis for which a right-of-use asset and liability are not recorded, in accordance with the practical expedient elected.

The Company has embedded leases in certain research and license agreements for which the Company has recorded a right of use asset and liability. These arrangements are not significant in comparison to the Company’s total operating lease assets and liabilities. In addition, the Company has identified certain short-term leases embedded within its manufacturing contracts which are not recorded on the Company’s balance sheet in accordance with the practical expedient elected.

The Company identified and assessed the following estimates in recognizing the right-of-use asset and corresponding liability:

The following table summarizes the lease assets and liabilities as of December 31, 2020 and 2019 (in thousands):

	As of December 31,
	2020		2019
Assets
Operating lease assets	$	50,865	$	41,502
Total lease assets		50,865		41,502
Liabilities
Current
Operating lease liabilities		11,362		8,489
Non-current
Operating lease liabilities, net of current portion		50,067		44,050
Total lease liabilities	$	61,429	$	52,539

The following table summarizes operating lease costs included in research and development and general and administrative expense, as well as sublease income for the twelve months ended December 31, 2020 and 2019 (in thousands):

	Years Ended December 31,
	2020			2019
Operating lease costs	$	14,342		$	8,067
Short-term lease costs		7,339			4,554
Variable lease costs		6,368			4,282
Sublease income		(587	)		(525	)
Net lease cost	$	27,462		$	16,378

The following table summarizes the maturity of undiscounted payments due under lease liabilities and the present value of those liabilities as of December 31, 2020 (in thousands):

	Total
2021	$	17,020
2022		13,815
2023		12,650
2024		10,761
2025		10,265
Thereafter		37,206
Total	$	101,717
Present value adjustment		(40,288	)
Present value of lease liabilities	$	61,429

The following table summarizes the lease term and discount rate as of December 31, 2020 and 2019:

	As of December 31,
	2020			2019
Weighted-average remaining lease term (years)
Operating leases		9.1			6.0
Weighted-average discount rate
Operating leases		9.3	%		9.9	%

The following table summarizes the cash paid for amounts included in the measurement of lease liabilities for the year ended December 31, 2020 and 2019 (in thousands):

	Years Ended December 31,
	2020			2019
Cash paid for amounts included in measurement of lease liabilities:
Operating cash flows used in operating leases	$	(13,161	)	$	(8,420	)
Operating lease non-cash items:
Right-of-use assets increased through lease modifications and reassessments		3,169			826
Right-of-use assets obtained in exchange for operating lease liabilities		13,956			18,088

In November 2019, the Company, together with one of its partners, based on foreign exchange rates as of December 31, 2020, committed to making $3.8 million in annual rental payments to a clinical manufacturing organization under a lease arrangement for a five-year period following commencement of the lease arrangement. The lease arrangement is expected to commence in the first quarter of 2021 and all payments will be split equally between the Company and its partner.