MRC1 and familial ovarian cancer: The prevalence of MMR‐deficiency or microsatellite instability (MSI) in familial ovarian cancer has been estimated between 10% and 20%.76, 84 Loss of MMR expression is more commonly found in non‐serous ovarian cancer, particularly endometrioid and clear cell carcinomas.85 Mean age at diagnosis in women with pathogenic germline MMR variants is 9 to 13 years earlier than the general population and cumulative lifetime risk of ovarian cancer has been reported as low as 3.7% (1.4%‐13%).86 Prognosis is affected by MMR variants.