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 expires in October 2021, with an option for an additional three-year term. 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 will terminate on the scheduled termination date of the original lease. Under the terms of the sublease, the subtenant is 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 is 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 the operating right-of-use asset upon adoption of the new leasing standards.

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

The following table summarizes operating lease costs, variable lease costs and sublease income:

	Three months ended March 31,
	2021			2020
	(in thousands)
Operating lease costs	$	623		$	623
Variable lease costs		459			528
Sublease income		(235	)		(228	)
Net lease cost	$	847		$	923

The maturities of our operating lease liabilities and minimum lease payments as of March 31, 2021 were as follows:

For the Years Ending December 31,	Amount (in thousands)
2021		2,863
2022		2,987
2023		3,077
2024		3,169
Thereafter		7,197
Total undiscounted lease payments	$	19,293
Less: imputed interest		(4,472	)
Present value of operating lease liabilities	$	14,821

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

	As of March 31, 2021
Weighted-average remaining lease term (years)
Operating leases		5.7
Weighted-average discount rate
Operating leases		10.0	%

The following table summarizes cash paid for amounts included in the measurement of the Company’s operating lease liabilities.

	Three months ended March 31,
	2021		2020
	(in thousands)
Cash paid for amounts included in the measurement of lease liabilities	$	965	$	943