The findings that PML tends to develop in idiopathic CD4+ T lymphocytopenia [34], that HIV-seropositive patients with PML lack CD4+ T cell responses to JCV [35], and PML patients with low numbers of CSF CD4+ T cells remain persistently JCV-positive in the CSF [36] collectively suggest the crucial role of CD4+ T cells as well as CD8+ cytotoxic T cells in the elimination of JCV and prevention of PML. This evidence concerns the gene CD8A and progressive multifocal leukoencephalopathy.