Human T-cell leukemia virus type 1 (HTLV-1), a human oncogenic retrovirus, is the causative agent of an aggressive CD4+ T-cell malignancy, adult T-cell leukemia/lymphoma (ATL/ATLL) [1–3] and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [4, 5]. This evidence concerns the gene CD4 and tropical spastic paraparesis.