BRCA1 is the major risk gene for breast cancer. However, not all women who carry harmful BRCA1 mutations (‘BRCA1 carriers’) develop breast cancer, and those who do can be diagnosed at very different ages, suggesting that additional factors influence the risk of BRCA1 carriers. This study analyzed the protein-coding genes of 321 Israeli women who all carried the same BRCA1 mutation. It found that BRCA1 carriers with an additional damaging DNA change in genes involved in the innate immune system, and especially in genes linked to the activity of a specific type of immune cells – natural killer – tended to develop breast cancer at a younger age. These findings suggest that immune-related genes may help explain differences in breast cancer risk and could improve personalized risk prediction for BRCA1 carriers. (https://jmg.bmj.com/content/early/2026/03/23/jmg-2025-111394)