Experiments using JPX-9 cells, which is a subline of the Jurkat HTLV-1-negative human T-cell leukemia cell line that expresses biologically active Tax protein under the control of the metallothionein promoter, showed that CCL1 was expressed exclusively in Tax-positive cells, suggesting that CCL1 expression was driven exclusively by HTLV-1 Tax at the single-cell level likely because of the Tax transactivation function. This evidence concerns the gene CCL1 and T-cell leukemia.