ABCG2 and cancer: Nowadays, the largest challenge for cancer drug treatment is MDR initiated by the elevation of the ATP-binding cassette (ABC) efflux transporters, including P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1) and breast cancer resistance protein (BCRP/ABCG2) (142, 143).