Since ceefourin-1 blocks the transport of structurally diverse MRP4 substrates as SN-38 (an irinotecan metabolite and topoisomerase 1 inhibitor [3]), E217bG, D-luciferin, cAMP, and 6-MP [12], we studied the antiproliferative and apoptotic effect of ceefourin-1 alone and in combination with 6-MP in both Jurkat T-ALL cells and normal lymphoblasts (CRL-1991) to determine if ceefourin-1 enhances the chemotherapeutic activity of 6-MP. This evidence concerns the gene ABCC4 and acute lymphoblastic leukemia.