ABCG2 and breast cancer: Resistant CEM/ADR5000 and sensitive CCRF-CEM leukemia cells;Colon cancer HCT116 (p53+/+) cells and t clone HCT116 (p53−/−);Glioblastoma U87MG cells and resistant U87MG.ΔEGFR cells;MDA-MB-231-pcDNA3 breast cancer cells and resistant subline MDA-MB-231-BCRP clone 23 cells;Hepatic carcinoma (HepG2)