ABCG2 and cancer: In previous literature, it has been discussed that the limitation of the exposure to anticancer drugs and drug resistance in cancer cells is related mainly to the overexpression of ATP-binding cassette (ABC) efflux transporters: P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) [25,26].