Human T-cell leukemia virus type 1 (HTLV-1) is a replication-competent human retrovirus associated with two distinct types of diseases: a malignancy of mature CD4+ T cells called adult T-cell leukemia-lymphoma (ATL) and a chronic inflammatory central nervous system disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Here, CD4 is linked to adult T-cell leukemia/lymphoma.