These responses include both CD4+ T-cell lymphoproliferation and CD8+ T-cell activity against the JCV VP-1 protein, and a systemic humoral immune response, as assessed by JCV enzyme-linked immunosorbent assay (ELISA) at PML diagnosis, and are significantly greater in HIV-PML survivors compared to non-survivors [50]. This evidence concerns the gene CD4 and progressive multifocal leukoencephalopathy.