ABCG2 and cancer: Studies have found that one of the mechanisms of MDR is the overexpression of ATP-binding cassette (ABC) superfamily of transporters in cancer cells, such as P-glycoprotein (P-gp/ABCB1), MDR-associated protein 2 (MRP2/ABCC2), and breast cancer resistance protein (BCRP/ABCG2) (Wu et al., 2014; Kathawala et al., 2015; Song et al., 2016).