MLH1 and Lynch syndrome: The heritable component of ovarian cancer is predominantly due to constitutional pathogenic variants in BRCA1 and BRCA2 with as many as 22% of women with high grade serous ovarian cancers (HGSOC) carrying pathogenic variants in these genes.4 The other leading heritable cause is Lynch syndrome (LS), an inherited mismatch repair (MMR) deficiency due to constitutional pathogenic variants affecting one of the four MMR genes, MSH2, MLH1, MSH6 and PMS2.