BRCA1 and breast cancer: To support our interpretation that the results can be generalized to the entire BRCA1/2 population, we highlight two opposing sources of bias: (1) excluding BRCA1/2 carriers tested after a breast cancer diagnosis may lead to an underestimation of breast cancer risk and (2) BRCA1/2 carriers with lower penetrance—who may never develop or may develop breast cancer later in life—are less likely to be referred for genetic testing, which could lead to an overestimation of breast cancer risk.