The mechanisms of chemoresistance in cancer cells are complicated and main mechanisms include (1) increased drug efflux mediated by membrane transport proteins such as ABC transport proteins family members p-glycoprotein (p-gp), multidrug resistance-associated protein (MRP), and breast cancer resistance protein (BCRP/ABCG2) [5, 6, 12]; (2) reduction in cell apoptosis mediated by bcl-2 family [7–9], NF-kB, and p53 [10]; and (3) other drug resistance mechanisms mediated by topoisomerase and glutathione S-transferase (GST). This evidence concerns the gene ABCG2 and cancer.