5—BUSINESS AND CREDIT CONCENTRATION

No customer accounted for 10% or more of the Company’s revenues for the years ended December 31, 1997, 1998 and 1999, respectively. The Company did have two customers that each had ending accounts receivable balances that were greater than 10% of the Company’s balance at December 31, 1999. The Company does not require collateral on accounts receivable or letters of credit on many foreign export sales. The Company periodically evaluates its customers’ creditworthiness before extending credit.

6—MARKETABLE SECURITIES

Marketable securities categorized as “available for sale” are carried at their fair value of $27.0 million, $5.9 million and $6.8 million at December 31, 1997, 1998 and 1999, respectively. The unrealized gain (loss) at December 31, 1999 is included as a component of accumulated other comprehensive loss within total stockholders’ equity. The unrealized gain (loss) was immaterial for 1997 and 1998. Proceeds and gross realized gains (losses) from sale of securities for the years ended December 31, 1997, 1998 and 1999, were, $32.7 million, $27.6 million and $16.4 million and $331,000, $185,000 and $0, respectively. At December 31, 1999, all marketable securities, which consist primarily of commercial paper, had a maturity date within one year.

7—INVENTORIES

Inventories consist of the following:

8—PROPERTY AND EQUIPMENT

Property and equipment consist of the following:

Depreciation and amortization expense was $3.0 million, $5.6 million and $6.6 million for the years ended December 31, 1997, 1998 and 1999, respectively.