Secondly, up to 25% of hereditary cases of the disease are caused by mutations in highly penetrant (BRCA1, BRCA2, PTEN, TP53, CDH1, STK11) (increase the risk of developing BC by up to 80%) and 2–3% in moderately penetrant (CHEK2, BRIP1, ATM, PALB2) (cause a 2-fold increased BC risk) genes [4]. The gene discussed is CHEK2; the disease is breast cancer.