High (e.g., due to a germline BRCA1 mutation8) and/or prolonged (e.g., hormone replacement therapy9,10) exposure to the sex steroid hormone progesterone, impacting receptor activator of nuclear factor-kB (RANK), wingless/integrated (WNT), and other signaling pathways, is a key driver for the development of breast cancers with the worst prognoses11–15. Here, BRCA1 is linked to breast cancer.