Recognition of extracellular, MHC-class II-presented viral antigens by CD4+-cells and subsequent activation of cytotoxic CD8+-cells appears to be important for the control of JCV-infected cells [11]–[14], and an association of HLA-class I haplotypes and CD8+-cellular responses with prognosis of PML has been reported [15], [16]. The gene discussed is CD4; the disease is progressive multifocal leukoencephalopathy.