ABCG2 and melanoma: Numerous members of ABC transporters have been described, although only a few are known to be expressed in human CSCs: multidrug resistance 1 (MDR1) or P-glycoprotein (Pgp)/ABCB1, multidrug resistance protein 1 (MRP1/ABCB1), breast cancer resistance protein (9BCRP), and the melanoma-associated, chemoresistance mediator, ABCB5.