Thus, the breast cancer susceptibility gene 1 (BRCA1), breast cancer susceptibility gene 2 (BRCA2), phosphatase and tensin homolog (PTEN), tumor protein 53 (TP53), cadherin 1 (CDH1), and serine/threonine kinase 11 (STK11/LKB1) are known as highly penetrant genes, while the cell cycle checkpoint kinase 2 (CHEK2), BRCA1 interacting helicase/protein 1 (BRIP1), ataxia-telangiectasia mutated gene (ATM), and partner and localizer of BRCA2 (PALB2) are known as moderately penetrating genes that suffer mutations, confering up to an 80% and 2–3% lifetime risk of BC, respectively [147]. Here, CHEK2 is linked to breast cancer.