Hereditary breast and ovarian cancer (HBOC) and Lynch syndrome comprise two common hereditary cancer syndromes associated with increased likelihood of OC; these are caused by pathogenic germline variants in BRCA1 and BRCA2 (BRCA1/2) and mismatch repair (MMR) genes including MLH1, MSH2, MSH6, and PMS2, respectively. This evidence concerns the gene MLH1 and Lynch syndrome.