The fact that HTLV-1 provirus accumulated in the CSF in HAM/TSP patients compared to HTLV-1 asymptomatic carriers [17,18] and that the main lesion of the spinal cord in HAM/TSP is in the lower thoracic cord (the watershed zone providing stagnant blood flow in hemodynamic condition) [19,20] might indicate that the outcome is induced by heightened transmigrating activity of HTLV-1-infected CD4+ T cells of HAM/TSP patients. This evidence concerns the gene CD4 and tropical spastic paraparesis.