Mismatch repair (MMR) deficiency is a genetic abnormality which arises due to the loss of one or more of the MMR proteins and increases the risk of developing several cancer types, including colorectal, endometrial and ovarian cancers.10 MMR deficient (dMMR) CC accounts for 15–20% of CCs11 and is distinct from MMR proficient (pMMR) CC with respect to various characteristics. The gene discussed is MRC1; the disease is ovarian carcinoma.