This high HTLV-1 Tax expression in HAM/TSP CD4+CD25+CCR4+ T cells (Foxp3−) and low HTLV-1 Tax expression in ATL CD4+CD25+CCR4+ T cells (Foxp3+) suggests that intracellular HTLV-1 expression may act as a “switch” that directs T cell plasticity from Foxp3+ Treg cells to IFN-γ+Foxp3− T cells. The gene discussed is FOXP3; the disease is tropical spastic paraparesis.