11. Commitments and Contingencies Leases In May 2006, the Company entered into a six-year operating lease for 23,494 square feet of office space. The Company received certain tenant improvement allowances and rent abatement and has an option to extend the lease for five years following the expiration of the initial term. Monthly rental payments are adjusted on an annual basis and the lease expires in September 2012. As security for the lease, a letter of credit in the initial amount of $1,581,000 was required by the landlord. The letter of credit is collateralized by a certificate of deposit in the same amount that is classified as restricted cash in the Company's balance sheets. The required amount subject to the letter of credit and corresponding certificate of deposit was eligible to be reduced by 22% on each of the first four anniversaries of the commencement of the lease and, as of June 30, 2011, the letter of credit and corresponding certificate of deposit had been reduced on each of the four anniversaries by an aggregate of $1,391,000. The value of the letter of credit and corresponding certificate of deposit, classified as restricted cash on the Company's balance sheets was $190,000 at June 30, 2011 and December 31, 2010, respectively. Rent expense under the Company's lease agreement for the three months ended June 30, 2011 and 2010 was $215,000 and $214,000, respectively. Rent expense under the Company's lease agreement for the six months ended June 30, 2011 and 2010 was $428,000 and $429,000, respectively. Corporate Credit Card In 2009, the Company entered into a pledge agreement pursuant to the establishment of a corporate credit card program whereby the Company pledged $150,000 in a certificate of deposit as collateral. During the six months ended June 30, 2011, the Company increased its pledged amount by $300,000 related to an increase in its credit limit. These funds are classified as restricted cash on the Company's balance sheets at June 30, 2011 and December 31, 2010, respectively. Supply Agreeme