Cash, Cash Equivalents and Short-term Investments

Cash, cash equivalents and short-term investments consisted of the following:

The Company's short-term investments have maturities of less than 12 months and are classified as available for sale. As of January 31, 2011 and 2012 the cost basis of the Company's cash and cash equivalents approximated their fair values and as a result, no unrealized gains or losses were recorded as of January 31, 2011 and 2012.

The following table summarizes the Company's available-for-sale securities' adjusted cost, gross unrealized gains, gross unrealized losses and fair value by significant investment category as of January 31, 2012. No such investments were held as of January 31, 2011.

The Company's investment policy requires investments to be investment grade, primarily rated "A1" by Standard & Poor's or "P1" by Moody's or better for short-term investments, with the objective of minimizing the potential risk of principal loss. In addition, the investment policy limits the amount of credit exposure to any one issuer.

The unrealized losses on our available-for-sale securities were primarily a result of unfavorable changes in interest rates subsequent to the initial purchase of these securities. As of January 31, 2012, the Company owned 13 securities that were in an unrealized loss position. The Company does not intend nor expect to need to sell these securities before recovering the associated unrealized losses. It expects to recover the full carrying value of these securities. As a result, no portion of the unrealized losses at January 31, 2012 is deemed to be other-than-temporary and the unrealized losses are not deemed to be credit losses. No available-for-sale securities have been in an unrealized loss position for 12 months or more. When evaluating the investments for other-than-temporary impairment, the Company reviews factors such as the length of time and extent to which fair value has been below cost basis, the financial condition of the issuer and any changes thereto, and the Company's intent to sell, or whether it is more likely than not it will be required to sell, the investment before recovery of the investment's amortized cost basis. During the fiscal year ended January 31, 2012, the Company did not recognize any impairment charges.

Accounts Receivable

Accounts receivable, net consisted of the following:

Property and Equipment

Property and equipment consisted of the following:

Depreciation and amortization expenses totaled $1.1 million, $1.6 million, and $4.5 million for the years ended January 31, 2010, 2011 and 2012, respectively. The Company wrote off net assets due to asset retirement totaling $0.3 million for the fiscal year ended January 31, 2012. There were no materials write-offs during the fiscal years ended January 31, 2010 and 2011.