Approximately 5–10% of all breast cancers are estimated to develop due to high-impact germline mutations in breast cancer susceptibility genes, with up to 30% due to pathogenic mutations in BRCA1 and BRCA2 and with a smaller proportion carrying mutations in other susceptibility genes, such as PTEN, TP53, CHEK2, PALB2 and STK112. Here, CHEK2 is linked to breast cancer.