ABCG2 and cancer: P-glycoprotein/multidrug resistance protein 1 (P-gp/MDR1/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) are the most significant ABC transporters clinically proven to be associated with multidrug resistance during cancer therapy.