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The death of a patient in a clinical trial run by Seattle-based Targeted Genetics is raising questions about a promising gene-therapy method until now thought to be safe.

The "adeno-associated virus" (AAV) used to deliver the company's drug for severe inflammatory arthritis has not previously caused any serious problems, so "no one knows why it happened," said Dr. David Russell, a professor of medicine at the University of Washington who specializes in gene therapy.

"We're all just waiting to see if it had anything to do with AAV," he said.

The tragedy is also a financial blow for Targeted, whose shares plummeted for a second day Friday and lost nearly 40 percent this past week.

The company reported Tuesday that a patient enrolled in experiments for its leading drug candidate had become seriously ill, and said the trial was being suspended as a precautionary measure. Two days later, the company and the U.S. Food and Drug Administration said the unidentified patient was dead.

Nothing else has been said of the patient's last moments, whereabouts or even cause of death. Both...