CD4 and tropical spastic paraparesis: The human T-cell leukemia virus (HTLV-1) is the etiologic agent of numerous diseases, including adult T-cell leukemia/lymphoma (ATLL)1, an aggressive CD4+ T-cell malignancy, and various inflammatory diseases, such as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP)2.