8. LONG‑TERM OBLIGATIONS

We currently lease our research and office space under a noncancelable lease agreement with our landlord, HCP BTC, LLC (formerly known as Slough BTC, LLC) which expires in 2018. The lease term provides for renewal option for up to two additional periods of five years each, and rental payments on a graduated scale. We determined our existing lease agreement to be an operating lease and recognize rent expense on a straight‑line basis over the lease period.

In December 2014, we entered into a sublease agreement with an unrelated third party to occupy a portion of our research and office space. We expect to receive over $5.0 million in future sublease income (excluding our subtenant’s share of facilities operating expenses) over the remaining term of the sublease. In connection with this sublease, we recognized a loss on sublease of $9.3 million during the fourth quarter of 2014.  We record rent expense on a straight-line basis for our lease, net of sublease income, wherein such arrangements contain scheduled rent increases over the term of the lease and sublease, respectively.  For our sublease arrangement which we classified as an operating lease, our loss on the sublease was comprised of the present value of our future payments to our landlord less the present value of our future rent payments expected from our subtenant over the term of the sublease.  Further, in conjunction with our facilities lease, we have previously issued to our landlord warrants to purchase our common stock.  We have previously capitalized the fair value of these warrants at issuance as part of our other long-term assets and they are being amortized over the term of our lease.  As a result of the sublease agreement that we entered into in December 2014, we included approximately $265,000 representing the unamortized portion of the warrant fair value attributable to the sublet space in the determination of our loss on sublease (see Note 9). The liability arising from this sublease agreement was determined using a credit-adjusted risk-free rate to discount the estimated future net cash flows. 

The changes in the liability related to the sublease agreement during the year ended December 31, 2015 were as follows (in thousands):

At December 31, 2015, future minimum lease payments and obligations under our noncancelable operating lease, net of sublease receipts, were as follows (in thousands):

Rent expense under our operating lease amounted to approximately $8.9 million (net of sublease income, subtenant’s share of certain facilities operating expense and amortization of deferred liability in the aggregate total of $6.3 million), $15.1 million and $14.8 million for the years ended December 31, 2015, 2014 and 2013, respectively.