Diseases resulting from HTLV-1 infection include an aggressive CD4+ T-cell malignancy called adult T-cell leukemia/lymphoma (ATL) [3,4,5] and a debilitating, progressive neurologic disorder called HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [6,7]. Here, CD4 is linked to tropical spastic paraparesis.