The report sent shares down as much as $6.94, or 17 percent, to $35 in extending trading on the Nasdaq Stock Market. The patient was diagnosed with progressive multifocal leukoencephalopathy, or PML, after 14 Tysabri infusions, Biogen of Cambridge, Massachusetts said today in a regulatory filing.

Biogen and its marketing partner, Irish drugmaker Elan Corp., said in August they were revising Tysabri's prescribing information to show the rare brain infection may occur in patients taking the drug as a sole treatment. Two cases of Tysabri patients with PML, which has no cure, were reported on July 31, the first since the drug was reintroduced in the U.S. in 2006. The companies pulled Tysabri from the market in February 2005 after three patients, two of whom died, contracted the illness.

``If people hadn't been expecting another case, they were naive,'' said Bret Holley, an analyst for Oppenheimer & Co., in a telephone interview today. ``I don't think this will negatively affect prescriptions any further.''

More than 48,000 patients have taken Tysabri, Biogen told investors on Oct. 21, when the company released its earnings. Chief Executive Officer James Mullen said 100,000 patients will be taking the drug by 2010. Tysabri generated worldwide sales of $236 million in the third quarter.

``We got confirmation of PML in a Tysabri patient today,'' said Naomi Aoki, a Biogen spokeswoman, in a telephone interview. ``She is currently under the care of her treating physician.''

Previous Cases

Aoki declined to comment on the patient's condition. One of the previous patients was continuing to receive outpatient treatment and was improving. The other remained hospitalized, Aoki said.

``The rate is consistent with our expectations, given the information that's in the label,'' Aoki said.

PML is included in Tysabri's prescribing information as a possible side effect in 1 of every 1,000 patients taking the drug. The condition occurs when the JC virus, named with initials for the first patient diagnosed with it, evades the body's immune defenses and penetrates the brain, causing irreversible damage.

``While a new Tysabri PML case clearly isn't good news, it should come as little surprise, given the established pattern,'' Christopher Raymond, an analyst with Robert W. Baird said today in an investment note.

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