BRCA1 and breast cancer: Importantly, in addition to the original floxed Trp53 allele, which was deleted by Cre, the wild-type (wt) Trp53 allele was also invariably lost in all tumors subjected to WES (Fig. 2a and Supplementary Data 1), indicating that the loss of p53 function is a prerequisite for mammary tumor development in mice following the inactivation of BRCA1/2 and PALB2.