A study performed by Yamano et al. [31] showed that persistent activation of the immune response, induced by Tax, in patients with HAM/TSP may be associated with a decrease in the expression of CD4+CD25+FOXP3+ T cells that possess a suppressive function and an accumulation of CD4+CD25+FOXP3- T cells that can exacerbate the pathogenic process of HAM/TSP. The gene discussed is CD4; the disease is tropical spastic paraparesis.