During chemotherapy treatment, certain cancer cells upregulate the expression of ATP-binding cassette (ABC) efflux transporters, including P-glycoprotein (P-gp/ABCB1), multidrug resistance protein 2 (MRP2/ABCC2), and breast cancer resistance protein (BCRP/ABCG2) [67]. This evidence concerns the gene ABCG2 and cancer.