ABCG2 and breast cancer: The main efflux transporters involved in BC MDR are ABCB1 (also termed P-glycoprotein, P-gp, or MDR1), multidrug resistance protein 1 (MRP1/ABCC1) and ABCG2 (also termed breast cancer resistance protein BCRP or mitoxantrone resistance protein MXR) [12, 13].