Commitments and Contingencies We lease our office and warehouse facilities under operating leases, which expire at various dates through 2021 . Our primary operating lease commitments at March 31, 2016 related to our headquarters in Santa Clara, California, our office in San Francisco, California, and our warehouse in Shepherdsville, Kentucky. We recognize rent expense on a straight-line basis over the lease period. Where leases contain escalation clauses, rent abatements, or concessions, such as rent holidays and landlord or tenant incentives or allowances, we apply them in the determination of straight-line rent expense over the lease term. On April 10, 2015, we signed an agreement to sublease effectively one half of our warehouse in Kentucky. On March 15, 2016, our subtenant expanded their sublease to lease additional space in our warehouse. We expect this sublease agreement to generate $0.1 million of sublease income per month through the end of November 2016. Rental expense, net of sublease income, was approximately $0.4 million and $0.9 million in the three months ended March 31, 2016 and 2015 , respectively. From time to time, third parties may assert patent infringement claims against us in the form of letters, litigation, or other forms of communication. In addition, we may from time to time be subject to other legal proceedings and claims in the ordinary course of business, including claims of alleged infringement of trademarks, copyrights and other intellectual property rights; employment claims; and general contract or other claims. We may also, from time to time, be subject to various legal or government claims, disputes, or investigations. Such matters may include, but not be limited to, claims, disputes, or investigations related to warranty, refund, breach of contract, employment, intellectual property, government regulation, or compliance or other matters. In July 2010, the Kentucky Tax Authority issued a property tax assessment of approximately $1.0 million related to our textbook library locate