ABCG2 and cancer: The overexpression of one or more multidrug resistance (MDR)-linked ATP-binding cassette (ABC) drug transporters, such as ABCB1 (P-glycoprotein, MDR1), ABCC1 (multidrug-resistance protein 1, MRP1), and ABCG2 (breast cancer resistance protein (BCRP), MXR), continues to pose a significant challenge to cancer chemotherapy [1,2,3,4].