HTLV-1 is the causative agent of two main diseases: adult T-cell leukemia/lymphoma (ATLL), a chemotherapy-resistant non-Hodgkin’s malignancy of CD4+ T cells, and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic, progressive neurogenerative disease (5). The gene discussed is CD4; the disease is tropical spastic paraparesis.