The biotech sector is taking a beating today after Affymetrix (AFFX) warned that slow sales would leave the company's financial results below analysts' expectations.

The group dropped in the wake of last night's preannouncement, as the Nasdaq Biotech Index dropped 5.4%, and the Dow Jones U.S. Biotechnology Index fell 4.6%.

Investors didn't take kindly to the warning from Affymetrix and sent the stock plummeting 33.6% today to $27.20. Last night, the company said it would post a loss of $4 million to $7 million in the second quarter, excluding noncash charges. Based on 57.6 million outstanding shares, that puts the company's loss at 6 cents to 12 cents a share. Analysts are looking for a loss of 2 cents, according to Thomson Financial/First Call. The company said the softening economy is "contributing to the unanticipated weakness in orders, making it increasingly difficult for the company to predict its financial performance with certainty at this time."

Adding fuel to the fire was news that the Food and Drug Adminstration won't approve a new drug application for a prostate cancer treatment developed by Amgen ( AMGN - Get Report) and Praecis Pharmaceuticals ( PRCS). The FDA said the application was "inadequate for approval," but the companies plan to meet with the agency to discuss any additional steps that might be taken to gain the green light.

Shares of Amgen lost 5.2% to $63.28, but Praecis plunged 37.8% to $14.

Other biotech players also felt the pain. Millennium Pharmaceuticals ( MLNM) dropped 9% to $32.95, Enzo Biochem ( ENZ) fell 7.8% to $29.13, and Idec Pharmaceuticals ( IDPH) lost 5.4% to $65.25. Celgene ( CELG) slid 5.9% to $29.87, while Gilead Sciences ( GILD) fell 3.2% to $48.10. Immunex ( IMNX) slipped 4.3% to $14.66.

Shares of Genome Therapeutics ( GENE) fell 7.8% to $12.03 in recent trading, while Abgenix ( ABGX) dropped 7.5% to $37.10. Gene Logic ( GLGC) tumbled 8.5% to $23.10.