Reply to "Mismatch repair and microsatellite instability—Recommendation for an optimal test strategy".

This document is a reply to a previous article discussing the significance of tumors with "unusual staining" from mismatch repair (MMR) immunohistochemistry (IHC) in endometrial cancer. The authors agree that ongoing conversation and multiple testing strategies are needed to improve outcomes for patients. They also emphasize the importance of quality assurance in performing and interpreting MMR IHC, including uniform fixation and grossing protocols. The authors acknowledge the potential benefits of primary assessment of MMR IHC staining on biopsy specimens but note that in their cohort, preoperative biopsies were performed outside their institution and MMR IHC was not performed on these specimens. They also discuss the interpretation of MSI testing in endometrial cancer and the use of the Promega panel for clarifying cases that are MSI-low on the National Cancer Institute panel. Further research is needed to define optimal testing for MMR-deficient/MSI endometrial cancer and predict response to immunotherapy. [Extracted from the article]