The increased CSF IL-15 we observed early in PML disease is consistent with inflammatory CNS disease [34]–[37] but is unlikely to be due solely to MS disease-associated inflammation [36]–[38] as the PML and non-PML groups both consisted of individuals with MS who were treated with natalizumab and had neurological symptoms consistent with PML. This evidence concerns the gene IL15 and progressive multifocal leukoencephalopathy.