Although CD4+CD25+CCR4+ T cells are predominantly infected by HTLV-1 in both HAM/TSP and ATL (Figure 6), it was demonstrated that the ratio of THAM cells (CCR4+Foxp3− with IFN-γ production) to Treg cells (CCR4+Foxp3+ with no cytokine production) in the CD4+CD25+CCR4+ T cell subset were high in HAM/TSP and low in ATL (Figure 2). Here, CD4 is linked to tropical spastic paraparesis.