HTLV-1 is the etiological agent of two life-threatening diseases: a malignancy of CD4+ T-cells termed adult T-cell leukemia/lymphoma (ATLL) [2,3], and an inflammatory disorder with progressive neurological disability and motor impairment known as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) [4,5]. Here, CD4 is linked to tropical spastic paraparesis.