High-risk pathogenic variants (PVs) in the currently known breast cancer-susceptibility genes BRCA1 (MIM: 113705), BRCA2 (MIM: 600185), and PALB2 (MIM: 610355) and intermediate-risk PVs in genes such as CHEK2 (MIM: 604373) and ATM (MIM: 607585)2, 3, 4, 5, 6, 7, 8, 9, 10 on average explain 20%–25% of the familial aggregation of breast cancer and much more at younger ages. Here, BRCA1 is linked to breast cancer.