ABCG2 and breast cancer: In the present study, the drug-resistant cancer cell lines included the P-gp-overexpressing CEM/ADR 5000 cells, the breast cancer resistance protein-expressing MDA-MB 231-BCRP cell lines, U87MG.ΔEGFR cells with mutation-activated EGFR gene (ΔEGFR) as well as the p53 knockout HCT116p53−/− cells [56–59].