Evaluation criteria of the ANRS 125 trial are the following. Clinical: rate of survival and functional score (Modified Rankin Outcome Scale) to M12. Virological: evolution of the JC viral load in the CSF ; and percentage of patients with JC virus clearance of the CSF to M3 and M6. Immunological: evolution of T CD4 and T CD8 subpopulations. Evolution of the anti-JC virus specific T cell (CD4 and CD8) responses. Pharmacological: dosage of the concentration of enfuvirtide in the CSF compared with the plasma.