Expression of the genes responsible for cytotoxic activities and apoptosis was reduced in HAM/TSP patients compared to ACs, and it appears that the survival of HTLV-1-infected T cells (HTLV-1-infected CD4+ T cells) stimulates and increases IFN-γ production, increases lymphocyte responses and leads to progression towards HAM/TSP. This evidence concerns the gene CD4 and tropical spastic paraparesis.