CD4 and tropical spastic paraparesis: Human T-cell leukemia virus type 1 (HTLV-1) is a human retrovirus associated with two distinct types of disease: a malignancy of mature CD4+ T cells called adult T-cell leukemia-lymphoma (ATL) [1–3] and a chronic inflammatory central nervous system disease, HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [4, 5].