Drug efflux from cancer cells is a common and important mechanism of resistance or multidrug resistance (MDR), which is inseparable from ATP-binding cassette (ABC) efflux transporters, including MDR-associated protein 2 (MRP2/ABCC2), P-glycoprotein (P-gp/ABCB1), and breast cancer resistance protein (BCRP/ABCG2) (15). The gene discussed is ABCG2; the disease is cancer.