BRCA1 and breast cancer: The CBC SIR was higher in BRCA1 PV carriers first diagnosed with BC at under age 45 years than at 45 years or over (under age 45 years: SIR, 23.5 [95% CI, 16.6 to 32.3]; 45 years or over: SIR, 9.31 [95% CI, 5.60 to 14.5]).