Tax, which is immunodominant, is subject to strong selection pressure from the autologous CTL response [11], and a high lytic efficiency of HTLV-1-specific CTLs (defined as the rate of clearance of Tax+CD4+ T cells/% CD8+ T cells/day) is associated with low PVL and a low risk of HAM/TSP [12]. This evidence concerns the gene CD4 and periventricular leukomalacia.