ABCG2 and cancer: Among the 49 ABC transporters known so far, P-glycoprotein (P-gp, MDR1, ABCB1), multi-drug resistance protein 1 (MRP1, ABCC1) and breast cancer resistance protein (BCRP, ABCG2) appear to play the most important roles in inducing resistance in several cancers, such as lung, breast, colon, ovarian, central nervous system (CNS) cancers, and melanomas [3,4,5].