HTLV-1 is another member of the Retroviridae family and the etiological agent of the aggressive CD4+ T cell proliferative malignancy, adult T-cell leukemia (ATL), as well as a progressive neurological disease known as HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP). This evidence concerns the gene CD4 and tropical spastic paraparesis.