Elan’s shares fell alongside those of Biogen Idec Inc. after cases of a potentially fatal brain disease linked to their multiple sclerosis drug, Tysabri, jumped to 23 from 13 previously disclosed. European regulators said they will review “any additional measures necessary to ensure the safe use of Tysabri.’’ Biogen Idec shares closed at $43.81 Friday, down nearly 11 percent for the week.
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