Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic human retrovirus that transforms CD4+ T-cells and causes a variety of diseases including adult T-cell leukemia/lymphoma (ATL) and a neurodegenerative disease called HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) (Uchiyama et al., 1977; Poiesz et al., 1980; Yoshida et al., 1982; Gessain et al., 1985; Osame et al., 1986). Here, CD4 is linked to tropical spastic paraparesis.