CD4 and tropical spastic paraparesis: HTLV-1 is the etiologic infectious agent of both adult T-cell leukemia/lymphoma (ATL), an aggressive and fatal disease of CD4+ T cells (Uchiyama et al., 1977; Poiesz et al., 1980; Yoshida et al., 1982), and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic inflammatory disease of the central nervous system (CNS) (Gessain et al., 1985; Osame et al., 1986).