ABCG2 and cancer: The significant contribution of thethree major ABC transporters, including ABCB1 (P-glycoprotein/P-gp/MDR1), ABCG2 (Breast Cancer Resistance Protein/BCRP/MXR), and ABCC1 (Multidrug Resistance Protein 1/MRP1) to MDR in cancer chemotherapy is well documented in vitro studies.