The Brief Reports on natalizumab were referred to Biogen Idec, the manufacturer, which offers the following response: After learning of one confirmed and one suspected case of progressive multifocal leukoencephalopathy (PML) in patients treated with natalizumab, Biogen Idec and Elan quickly notified the Food and Drug Administration (FDA) and other regulatory authorities. We worked closely with the FDA to understand the significance of these findings and to determine the appropriate action. On February 28, we voluntarily suspended all dosing and marketing of natalizumab; swift and decisive action was guided by our commitment to patient safety. Immediate efforts also included a . . .