ABCG2 and cancer: Cancer cells may rapidly acquire MDR, mainly due to the presence of adenosine triphosphate-binding cassette (ABC) transporters, such as the breast cancer resistance protein (BCRP/ABCG2) and P-glycoprotein (P-gp/MDR1/ABCB1) [4] as well as the oncogene epidermal growth factor receptor (EGFR) [5, 6, 16] and the deletions or inactivation of tumor suppressor gene p53 [17].