The high frequency of a T cell subset with immunostimulatory properties [48] in HAM/TSP patients, but not in asymptomatic carriers, suggests that CD39+CD25− CD4+ T cells contribute to the increased CD4+ T-cell proliferation seen in HAM/TSP despite concomitant expansion of CD4+ T cells with suppressive phenotype (CD39+CD25+). This evidence concerns the gene CD4 and tropical spastic paraparesis.