Thus, in a CD4+CD25+CCR4+ T cell population that mainly consists of suppressive T cell subsets such as Treg and Th2 under healthy conditions, IFN-γ-producing Foxp3−CD4+CD25+CCR4+ T cells, rarely encountered in healthy individuals, were increased in number and overproduced IFN-γ in HAM/TSP patients (Figure 2). The gene discussed is IFNG; the disease is tropical spastic paraparesis.