The relation between MRP4 overexpression in 6-mercaptopurine (6-MP) resistance was reported in T-lymphoblastic leukemia (T-ALL) cells (CEM cells), where MRP4 was up-regulated in tumorous cells but influx transporters were down-regulated, leading to a high efflux of 6-MP and survival of resistant cells [2]. The gene discussed is ABCC4; the disease is acute lymphoblastic leukemia.