Dynavax Technologies' ( DVAX) shares plummeted Monday after the company said clinical trial conditions made it impossible to tell whether its ragweed allergy treatment worked.

The company said that results from the trial were not clinically significant and efficacy measures were substantially lower in the latest trial than what has been observed in prior trials. Shares sank 24.5% to $6.45.

Shares of drug developer BioCryst Pharmaceuticals ( BCRX) fell 3.8% to $11.36 after the company appointed Jon Stonehouse its new chief executive. Founder and former CEO Dr. Charles Bugg retired.

St. Jude Medical ( STJ) surged 6.1% to $37.12 early Monday after guiding higher on fourth-quarter sales.

The medical-device maker said it expects to meet analysts' estimates of 39 cents a share for the fourth quarter and sales should come in at $864 million, above the expected $843 million. Defibrillator sales should come in on the high end of the company's estimates, while sales of its atrial fibrillation, neuromodulation, cardiology and cardiac surgery products should exceed expectations, the company says.

Medical technology company Affymetrix ( AFFX) said it expects fourth-quarter revenue to reach $100 million, compared with $99.47 million expected by analysts surveyed by Thomson First Call. Shares gained 16.3% to $25.27.

Shares of Seattle Genetics ( SGEN) jumped 21.7% to $6.44 after the company announced a license agreement with biotech giant Genentech ( DNA) to develop a blood cancer drug dubbed SGN-40. The treatment is proposed for patients with multiple myeloma, chronic lymphocytic leukemia and non-Hodgkin's lymphoma.

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