Since germ-line deletion of Brca1 proved to be lethal during embryogenesis, and its heterozygous loss did not initiate tumorigenesis [34,35,36], development of genetically engineered mouse models of Brca1-related cancer required tissue-specific conditional knockout systems based on the cytokeratin 14 (CK14), β-lactoglobulin, MMTV-LTR or WAP driven expression of the Cre recombinase [37,38,39,40]. This evidence concerns the gene BRCA1 and cancer.