13. Commitments and Contingencies Operating Leases In May 2013, the Company entered into a lease for office space in Burlington, Massachusetts. The lease is for a 42-month term with minimum monthly lease payments beginning at $17,588 per month and escalating over the lease term. The Company provided a letter of credit to the lessor in the amount of $98,000 as a security deposit pursuant to the lease agreement to secure its obligations under the lease. During 2015, this letter of credit was reduced to $50,000 pursuant to the original lease agreement. In July 2015 the Company entered into a first amendment to its existing lease for approximately 4,700 square feet of additional office space (the Additional Space) in Burlington, Massachusetts, as well as approximately 6,700 square feet of temporary space to be leased prior to the delivery of the Additional Space (which is anticipated to be delivered on May 1, 2016). The amendment extended the term of the original lease through October 31, 2019, contemporaneous with the Additional Space, and also provided the Company with an option to lease an additional 5,400 square feet of office space (the Option Space). On September 30, 2015, the Company exercised its option for the Option Space. In addition, the Company has the option to extend the term of a portion or the entire lease space for one additional three-year period. The Company may terminate the amendment for convenience with nine months notice upon the occurrence of certain events connected to its clinical stage programs. The total cash obligation for the base rent from inception through the lease termination date for the office space committed to under the original lease, and the amendment including the Option Space, is approximately $2,900,000. In addition to the base rent, the Company is also responsible for its share of operating expenses and real estate taxes. The Company incurred rent expense of $373,202, $242,946, and $227,381 for the years ended December 31, 2015, 2014 and 2013, respectively. Future min