BRCA1 and BRCA2 genes play a crucial role in repairing damaged DNA. Mutations in BRCA1 and BRCA2 substantially increase the likelihood of breast and ovarian cancer, respectively. This new research illustrates how small breaks in one strand of DNA can expand into a large single-stranded DNA gap that kills drug-resistant breast BRCA mutant cancer cells. The novel vulnerability in mutant BRCA1 and BRCA2 may be a potential target for new therapeutics.