In August 2017, we entered into a Lease Agreement (the Mission Bay Lease) with ARE-San Francisco No. 19, LLC (ARE) and terminated our sublease with Pfizer, Inc., effectively extending our lease term from 2020 to 2030 for our 134,356 square foot corporate office and R&D facility located at 455 Mission Bay Boulevard, San Francisco, California (the Mission Bay Facility).

The term of the Mission Bay Lease commenced on September 1, 2017, and will expire January 31, 2030, subject to our right to extend the term of the lease for two consecutive five-year periods. The monthly base rent for the Mission Bay Facility will escalate over the term of the lease at various intervals. During the term of the Mission Bay Lease, we are responsible for paying our share of operating expenses specified in the lease, including insurance costs and taxes. The Mission Bay Lease also obligates us to rent from ARE a total of an additional approximately 18,000 square feet of space at the Mission Bay Facility at specified delivery dates. The Mission Bay Lease includes various covenants, indemnities, defaults, termination rights, security deposits and other provisions customary for lease transactions of this nature.

In May 2018, we entered into a Lease Agreement (the Third Street Lease) with Kilroy Realty Finance Partnership, L.P. to lease 135,936 square feet of space located at 360 Third Street, San Francisco, California (the Third Street Facility) from June 2018 to January 31, 2030. An initial 1,726 square feet was delivered in June 2018, and a total of 67,105 square feet for two additional spaces was delivered in December 2018. The remaining space will be delivered in phases during 2019. The Third Street Lease will provide us additional facilities to support increased personnel for our San Francisco-based R&D activities. The lease term includes a right to extend the term for a consecutive five-year period. Our fixed annual base rent on an industrial gross lease basis includes certain expenses and property taxes paid directly by the landlord and will escalate each year over the term at specified intervals. We have a onetime right of first offer with respect to certain additional rental space at the Third Street Facility. The Third Street Lease includes various covenants, indemnities, defaults, termination rights, security deposits and other provisions customary for lease transactions of this nature.

We recognize rent expense on a straight-line basis over the lease period. For the years ended December 31, 2018, 2017 and 2016, rent expense totaled approximately $8.1 million, $4.7 million and $3.2 million, respectively. As of December 31, 2018 and 2017, we had total deferred rent balances of $9.3 million and $6.0 million, respectively, which we present in other long-term liabilities in our Consolidated Balance Sheets.

Our future minimum lease payments for our operating leases at December 31, 2018 are as follows (in thousands):

Year ending December 31,
2019	$	7,914
2020		10,617
2021		13,649
2022		14,117
2023		14,599
2024 and thereafter		98,315
Total future minimum lease payments	$	159,211