Sensitive and drug-resistant human cancer cell lines:Sensitive CCRF-CEM and multidrug-resistant P glycoprotein-overexpressing CEM/ADR5000; Wild-type HCT116 (p53+/+) and knockout HCT116 (p53−/−) colon cancer cells;Breast cancer cells transduced with control vector (MDAMB-231-pcDNA3) or with cDNA for the breast cancer resistance protein BCRP (MDA-MB-231-BCRP clone 23)Wild-type U87MG cells and U87MG glioblastoma multiforme cells transfected with an expression vector harboring an epidermal growth factor receptor (EGFR) gene with a genomic deletion of exons 2 through 7 (U87MG.ΔEGFR). This evidence concerns the gene ABCG2 and breast cancer.