Commitments and Contingencies

Operating Leases

We lease our facility under a non-cancellable operating lease for which we exercised our option to renew for an additional five-year period in fiscal 2015. The lease expires in August 2021.

Rent expense was $0.5 million, $0.5 million and $0.4 million during the years ended December 31, 2017, 2016 and 2015, respectively. At December 31, 2017, deferred rent aggregated $0.2 million, which is included in both current and noncurrent liabilities in the accompanying consolidated balance sheets. At December 31, 2017, the future minimum annual obligations under non-cancellable operating lease commitments are as follows:



Years Ending December 31, (in thousands)
2018	$	550
2019	569
2020	589
2021	402
2022	—
Thereafter	—
Total minimum payments required	$	2,110

License Agreements

We have entered into collaborative license agreements that provide us with rights to use certain know-how, technology and patent rights maintained by the licensors in our research and development efforts. Terms of the license agreements may require us to make upfront payments, milestone payments upon the achievement of certain product research and development objectives and royalty payments on fees received under our sub-licensing arrangements and/or future sales, if any, of commercial products resulting from the collaboration.

Certain of the licensing agreements require guaranteed minimum annual payments. Terms of the licensing agreements generally range from the remaining life of the patent up to 17 years and, in some cases, may be subject to earlier termination by either party upon specified circumstances.

Total expense incurred under all collaborative licensing agreements for upfront, milestone and royalty payments were $0.5 million, $0.3 million and $0.2 million during the years ended December 31, 2017, 2016 and 2015, respectively. Total cash paid under these agreements was $0.6 million during the year ended December 31, 2017 and $0.2 million during each of the years ended December 31, 2016 and 2015.

Future minimum annual cash obligations under all such license agreements will be $0.2 million in aggregate during 2018, and thereafter. These obligations are payable through ten years from the first commercial sale, if any, or expiration of the last patent to expire, the dates of which are not determinable at this time.

Letter of Credit

At December 31, 2017 and 2016, we were contingently liable for a standby letter of credit issued by a commercial bank for $60,000 for security on our lease. A restricted cash account with these amounts was held as cash collateral for the letter of credit.

Litigation

We are, from time to time, involved in legal proceedings, regulatory actions, claims and litigation arising in the ordinary course of business. Currently, we are not a defendant in any lawsuit.