Because hormonal chemoprevention has been controversial in women with BRCA1 mutations, the ability of metformin to impair both the metabolic rewiring and tumor-initiating capacity of BRCA1 haploinsufficient breast epithelial cells in vitro might suggest new avenues for testing metformin-based chemoprevention strategies in BRCA1 mutation carriers. This evidence concerns the gene BRCA1 and neoplasm.