Among these transmembrane proteins, the cellular protein P-glycoprotein (P-gP/MDR1/ABCB1), the multidrug resistance proteins (MRPs), and the breast cancer resistance protein (BCRP/ABCG2) mediate classic multidrug resistance (MDR) in cancer cells by functioning as an energy-driven efflux pump. The gene discussed is ABCG2; the disease is cancer.