HTLV-1 is the etiological agent of two major diseases: adult T-cell leukemia (ATL), a disease characterized by malignant proliferation of CD4+ T-lymphocytes, and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM), a neurodegenerative condition [3,4]. The gene discussed is CD4; the disease is tropical spastic paraparesis.