CD4 and tropical spastic paraparesis: HTLV-1 is the pathogenic agent of a severe form of leukemia/lymphoma designated Adult T-cell Leukemia/Lymphoma (ATL) characterized by the malignant transformation of CD4+ T cells [2] and of a severe neurological disorder designated HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic progressive neuromyelopathy characterized by spastic paraparesis, sensory dysfunction and sphincter function defects [3, 4].