ABCG2 and cancer: Cancer cells rapidly develop chemoresistance, mainly due to the presence of adenosine triphosphate-binding cassette (ABC) transporters [2–4], such as the breast cancer resistance protein (BCRP/ABCG2) and P-glycoprotein (P-gp/MDR1/ABCB1) [2] as well as the oncogene epidermal growth factor receptor (EGFR) [3, 4, 27] and the deletion or inactivation of tumor suppressor gene p53 [5].