HTLV-1 is the cause of a severe form of leukemia affecting adult CD4+ T cells (adult T-cell leukemia/lymphoma or ATL) and of a progressive neurological pathology designated HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), characterized by spastic progressive limb paralysis, sensory dysfunction, and sphincter function defects (Gessain et al., 1985; Osame et al., 1986). The gene discussed is CD4; the disease is tropical spastic paraparesis.