Elan Corp. plc (ELN) and Biogen Idec Inc. (BIIB) gapped open lower on Friday and held a range for the rest of the day.
The move followed news of two confirmed cases of progressive multifocal leukoencephalopathy or PML in multiple sclerosis or MS patients treated with Tysabri in the commercial setting.
Elan ended the session at $9.93 down $10.12 from Thursday's close. The stock closed at a new multi-year low.
Biogen ended the day at $50.01 down $19.75 from Thursday's close, its lowest level in over a year.
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