Human T cell leukaemia/lymphoma virus type I (HTLV-1), the first human oncoretrovirus to be discovered [1], causes a lymphoproliferative malignancy of CD4-activated cells called adult T-cell leukaemia/lymphoma (ATL) and a chronic myelopathy called tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM) [4]. This evidence concerns the gene CD4 and tropical spastic paraparesis.