Patients will be included on the following criteria : HIV-1 documented by Western Blot, clinical and radiological (MRI) evidence of active LEMP with clinical evolution (or deterioration) for less than 90 days, documentation of PML diagnosis for less than 30 days at the inclusion, informed consent (patient or confidence surrogate if decision making incapacity). Exclusion criteria will be the following: age less than 18-year-old ; concomitant opportunistic infection of the central nervous system; pregnancy - feeding; co-infection by the HIV2; history of immunotherapy (interleukin 2, alpha-interferon) or of treatment by FUZEON®; history of treatment by cidofovir; contra-indication to receive FUZEON ®.