BRCA1 and breast cancer: In breast cancer, mutations in BRCA1 or BRCA2 genes account for ~5% of all breast cancers and 15–20% of all hereditary cancers.9, 13 Approximately 55–65% of women who inherit a BRCA1 gene mutation and 45% of women who inherit a BRCA2 gene mutation will develop breast cancer before the age of 70 years.9