Further studies revealed that high HTLV-1 PVL was detected in CD4+CD25+CCR4+ T cells and the frequency of IFN-γ-producing CD4+CD25+CCR4+ T cells was dramatically increased in HAM/TSP patients, which was found to be correlated with disease activity and severity (Yamano et al., 2009; Araya et al., 2014). This evidence concerns the gene CD4 and tropical spastic paraparesis.