Infection with human T-cell leukemia virus type 1 (HTLV-1), an oncogenic retrovirus, is associated with the development of adult T-cell leukemia (ATL), an aggressive and lethal malignancy of CD4+ T lymphocytes and a chronic neuroinflammatory disease termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The gene discussed is CD4; the disease is tropical spastic paraparesis.