BCRP is physiologically expressed in the gastrointestinal tract, liver, kidney, brain, endothelium, mammary tissue, testis and placenta; moreover, overexpression of this transporter was reported in untreated solid tumors and in different types of leukemia[72], where the pump exhibits a very broad specificity for substrates like P-gp and MRP1. This evidence concerns the gene ABCG2 and leukemia.