A longer follow-up period would be of interest but may pose some hazard since the risk of PML increases with duration of NAT treatment.23 Another limitation is that the groups were not completely matched in the present study since 14 patients in the IFNβ subgroup only had sufficient serum material left from samples obtained during IFNβ therapy but not before and during NAT therapy. This evidence concerns the gene IFNB1 and progressive multifocal leukoencephalopathy.