It has been three decades since the discovery of human T-cell leukemia virus type 1 (HTLV-1), which is the first human retrovirus etiologically associated with an aggressive malignancy of CD4+ T cells known as adult T-cell leukemia (ATL) and a disabling chronic inflammatory disease of the central nervous system known as HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Here, CD4 is linked to tropical spastic paraparesis.