Some ABC transporter members, particularly ABCB1 (P-glycoprotein/MDR1), ABCC1 (Multidrug Resistance-associated Protein 1/MRP1) and ABCG2 (Breast Cancer Resistance Protein/BCRP) act as cell membrane pumps that are capable of extruding drugs from cancer cells, and their contribution to multidrug resistance is widely recognized in different tumors (for a review, see [2]). This evidence concerns the gene ABCG2 and cancer.