Advances in elucidating the molecular basis of the MDR phenotype have indicated that elevated membrane expression of drug efflux pumps such as P-glycoprotein (Pgp or ABCB1), multidrug resistance protein 1 (MRP1 or ABCC1), and ABCG2 is a frequent cause of MDR in human cancers [47]. The gene discussed is ABCG2; the disease is cancer.