To facilitate mammary tumor formation, we chose to co-delete one copy of Trp53. As the floxed alleles (Brca1f5-13, Brca2f11, Palb2f2-3, and Trp53f2-10) and the Wap-cre allele were from different genetic backgrounds, with elements of C57BL/6, 129sv, and FVB-N, we first conducted a multi-step crossing of the source mice to generate several “common ancestor” mice carrying all five alleles. This evidence concerns the gene TP53 and breast cancer.