However, approximately 5–10% of HTLV-1 carriers develop serious diseases, such as adult T cell leukemia/lymphoma (ATL), an aggressive malignancy of CD4+ T cells, and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a debilitating neuroinflammatory condition [3,4]. Here, CD4 is linked to tropical spastic paraparesis.