The high frequencies of CD39+CD25+CD4+ T cells in AC and HAM/TSP subjects reinforce the idea of induced Treg differentiation in HTLV-1 infection while the high frequency of CD39+CD25− CD4+ T cells only in HAM/TSP patients may contribute to the proinflammatory milieu seen in HAM/TSP and represent a marker of disease progression. The gene discussed is ENTPD1; the disease is tropical spastic paraparesis.