Although most CD4+CD25+CCR4+ T cells are infected with HTLV-1 in both HAM/TSP and ATL patients [54,56], 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 has been found to be high in HAM/TSP patients but low in ATL patients [54]. Here, IFNG is linked to tropical spastic paraparesis.