During the course of chemotherapy with anti-cancer drugs, cancer cells often acquire multidrug resistance via increased expression of efflux transporters such as P-glycoprotein (P-gp, ABCB1), multidrug resistance-associated proteins (MRPs, ABCCs), and breast cancer resistance protein (BCRP, ABCG2), thereby reducing the intracellular accumulation of anticancer drugs [8,9,10]. Here, ABCG2 is linked to cancer.