Among the CD4+ T helper cell populations that play a central role in adaptive immune responses, the CD4+CD25+CCR4+ T cell population, which primarily consists of suppressive T cell subsets, such as the Treg and Th2 subsets, in healthy individuals, is the predominant viral reservoir of HTLV-1 in both ATL and HAM/TSP patients. The gene discussed is CD4; the disease is tropical spastic paraparesis.