HTLV-1 is the causative infectious agent of both an aggressive and fatal non-Hodgkin’s CD4+ T-cell lymphoma called ATL and a chronic, progressive neurodegenerative disease termed HAM/TSP (Uchiyama et al., 1977; Poiesz et al., 1980; Yoshida et al., 1982; Gessain et al., 1985; Osame et al., 1986). The gene discussed is CD4; the disease is tropical spastic paraparesis.