Decreases in CD4, but not CD8, T cells are seen in the CSF of natalizumab-treated MS patients and CD4 T cell epitope-escape JCPyVs have been found in PML patients (Jelcic et al., 2016; Schneider-Hohendorf et al., 2014; Stüve et al., 2006). The gene discussed is CD4; the disease is progressive multifocal leukoencephalopathy.