Genetic testing for hereditary breast and ovarian cancer often finds DNA changes whose clinical meaning is unclear, leaving patients without clear guidance. In this study, we focused on a BRCA2 gene variant found mainly in Japanese people that has long been classified as a “variant of uncertain significance.” By combining nationwide family data with multiple independent laboratory tests, we show that this variant clearly disrupts BRCA2 function and should be considered disease-causing. This reclassification is important because it allows affected patients to access targeted treatments, such as PARP inhibitors, and enables more accurate genetic counselling for families. Our work highlights the importance of population-specific genetic evidence in precision medicine. (https://jmg.bmj.com/content/early/2026/03/19/jmg-2025-111305)