ABCG2 and cancer: When expressed at high levels, proteins like P-glycoprotein (ABCB1, P-gp) [9], the breast cancer resistance protein (ABCG2, BCRP) [10], and/or the multidrug resistance associated protein 1 (ABCC1, MRP-1) [11], have the ability to remove most of the approved cancer chemotherapeutics from the cells, making chemotherapies ineffective.