BRCA1 and breast cancer: A mutation in either of these two genes has been found to significantly increase the risk of breast and ovarian cancer development, with an approximately 72% (95% confidence interval (CI) 65–79) or 69% (95% CI 61–77) cumulative lifetime risk (up to the age of 80 years) for breast cancer for BRCA1 or BRCA2 mutation carriers, respectively.