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