HTLV-1 is the etiological agent of adult T-cell leukemia/lymphoma (ATL), an aggressive and lethal malignancy of CD4+ T lymphocytes, as well as a variety of autoimmune and inflammatory diseases including the neurodegenerative disorder tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). Here, CD4 is linked to tropical spastic paraparesis.