Using mouse models, we and others have demonstrated that complete knockout of Brca1 in the whole body (Brca1−/−) causes lethality at embryonic day 7–8 (E7-8)12,13; in contrast, mammary-specific deletion of exon 11 of Brca1 (Brca1Co/Co;MMTV-Cre) results in mammary tumour formation accompanied by massive genomic alterations and cellular lethality14. This evidence concerns the gene BRCA1 and breast cancer.