To further investigate the biological basis of this peculiar subcellular distribution of HBZ compared with the prominent nuclear localization found in ATL cells, we took advantage of an IL-2-dependent HTLV-1-immortalized T cell line, designated CB-CD4/HTLV-1 established from a HAM/TSP patient (Ozden et al., 2004). This evidence concerns the gene CD4 and tropical spastic paraparesis.