Among women with a BRCA1 mutation, the risk of breast cancer increased by 1.2-fold for each first-degree relative with breast cancer before age 50 years (HR=1.21; 95% CI 0.94–1.57), and among women with a BRCA2 mutation, the risk of breast cancer increased by 1.7-fold for each first-degree relative with breast cancer (HR=1.65; 95% CI 1.00–2.71). Here, BRCA1 is linked to breast cancer.