Human T-cell leukemia virus type 1 (HTLV-1) is etiologically associated with adult T-cell leukemia (ATL), an aggressive and lethal malignancy of CD4+ T cells, and with HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [1, 2]. This evidence concerns the gene CD4 and tropical spastic paraparesis.