Operating Leases—In September 2016, the Company entered into a noncancelable operating lease beginning in November 2016 for office, laboratory and manufacturing space in Bedford, Massachusetts, that expired in October 2021, with an option for an additional three-year term that was not exercised. In 2018, the Company entered into a sublease agreement for the entire leased premises. The rent commencement date of the sublease was December 2018, and the sublease terminated on the scheduled termination date of the original lease. Under the terms of the sublease, the subtenant was obligated to pay the Company aggregate base rent of approximately $2.7 million over the term of the sublease, based on the same level of rent the Company was obligated to pay the landlord, in addition to a passthrough of operating expenses and real estate taxes charged by the landlord.

In December 2017, the Company entered into a noncancelable operating lease for approximately 67,000 square feet of research and development, manufacturing and general office space in Bedford, Massachusetts. The lease expires in February

2027 with an option for an additional five-year term. Rent became due under the lease in two phases; rent on the first 46,000 square feet started in September 2018 and rent on the remaining 21,000 square feet started in March 2019. The initial annual base rent was $39.50 per square foot and increases by three percent annually. The Company is obligated to pay, on a pro-rata basis, real estate taxes and operating costs related to the premises. The lease agreement entered into in December 2017 allowed for a tenant improvement allowance not to exceed $10.9 million, which the Company received in full, to be applied to the total cost of tenant improvements to the leased premises. The unamortized balance of the tenant improvement allowance was included in deferred rent incentives and has been recorded as a reduction to operating right-of-use asset upon adoption of the new leasing standards.

In November 2021, the Company entered into an amendment of its December 2017 lease agreement (the “Lease Amendment”) for its corporate headquarters in Bedford, Massachusetts. The Lease Amendment increases the space under lease by approximately 23,011 square feet (the "Expansion Premises") and extends the expiration date of the existing premises under the lease from February 2027 to June 2030. The payment term with respect to the Expansion Premises commences on the earlier of (i) the date of the Substantial Completion of the Tenant’s Work (as both terms are defined in the Lease Amendment), (ii) the Company’s occupancy of any portion of the Expansion Premises, and (iii) May 1, 2022, and continues for a period of ten years and five months. The term of the Expansion Premises and the existing premises are not coterminous. Annual base rent for the existing premise under the Lease Amendment is approximately $4.7 million beginning on March 1, 2027, and increases by three percent annually; annual base rent for the Expansion Premises is approximately $1.4 million per year and increases by three percent annually. The Lease Amendment allows for a tenant improvement allowance not to exceed $5.3 million. The Lease Amendment was accounted for as a lease modification and the right-of-use asset and lease liability for the existing premises were remeasured at the modification date resulting in an increase of $10.9 million to both the right-of-use asset and lease liabilities.

The Company maintains letters of credit, secured by restricted cash, for security deposits totaling $2.0 million and $1.3 million as of December 31, 2021 and 2020, respectively, in conjunction with its current leases.

The following table summarizes operating lease costs and variable lease costs, as well as sublease income for the year ended December 31, 2021:


	Years ended December 31,
	2021			2020
	(in thousands)
Operating lease costs	$	2,592		$	2,492
Variable lease costs		2,127			2,355
Sublease income		(861	)		(913	)
Net lease cost	$	3,858		$	3,934

The maturities of our operating lease liabilities as of December 31, 2021 were as follows:


	Amount
For the Years Ending December 31,	(in thousands)

2022		3,326
2023		4,444
2024		4,578
2025		4,715
Thereafter		31,123
Total undiscounted lease payments	$	48,186
Less: imputed interest		(17,932	)
Less: estimated lease incentives		(6,320	)
Present value of operating lease liabilities	$	23,934

The following table summarizes the lease term and discount rate as of December 31, 2021:


	As of December 31, 2021
Weighted-average remaining lease term (years)
Operating leases		8.9
Weighted-average discount rate
Operating leases		10.5	%

The following table summarizes the supplemental cash flow information related to the Company's operating leases for the year ended December 31, 2021.


	Years ended December 31,
	2021		2020
	(in thousands)
Cash paid for amounts included in the measurement of lease liabilities	$	3,810	$	3,804
Increase in lease liabilities and right-of-use assets due to lease remeasurement	$	10,901	$	—