I then describe the human oncogenic retrovirus HTLV-1 (human T lymphocyte virus type 1) and its etiologically associated diseases, adult T-cell leukemia/lymphoma (ATLL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), with particular attention on the expression of CCR4 on HTLV-1-infected T cells and malignant T cells. Here, CCR4 is linked to adult T-cell leukemia/lymphoma.