BRCA1 and breast cancer: BRCA1 also regulates p53-dependent gene expression,75 and cooccurrence of somatic TP53 PVs is more commonly observed with BRCA1 PVs, compared with BRCA2 PVs.76 Furthermore, mammary-specific deletion of Tp53 and Brca1 leads to the development of murine mammary tumors having genomic and transcriptomic similarities to human basal-like BC.77 These potential biologic mechanisms linking BRCA1 to lobule expansion during pregnancy may offer insights into why the peak risk window for poor prognosis in PPBC among BRCA1 PV carriers was observed later.