Using a monoallelic BRCA1 germline mutation model that accurately mimics the molecular events of rapid and tissue-specific predisposition of breast cancer development associated with BRCA1 haploinsufficiency, we demonstrate the ability of denosumab to robustly impair tumorsphere forming ability, a functional marker that correlates with CSC numbers in cancer cell lines and with progenitor activity in nontransformed mammary epithelial cells of BRCA1mut/+ breast epithelial populations. Here, BRCA1 is linked to breast cancer.