Numerous studies indicate that the members of the ABC transporter family associated with multidrug resistance (MDR) in cancer cells include p-glycoprotein (P-gp/ABCB1), MRP1/ABCC1, MRP2/ABCC2, MRP4/ABCC4, and BCRP/ABCG2 [20] (Fig. 1). This evidence concerns the gene ABCG2 and cancer.