MSI-H / dMMR (microsatellite instability-high / mismatch-repair deficiency)
A tumor state marked by genomic instability from defective DNA mismatch repair, detected as high microsatellite instability (MSI-H) or loss of mismatch-repair proteins (dMMR).
MSI-H/dMMR is a tumor tissue characteristic rather than a single gene. It denotes genomic instability arising when a tumor's DNA mismatch-repair system is defective, producing an accumulation of changes in short repeated DNA sequences called microsatellites. Per the NCI Thesaurus, microsatellite instability is "genomic instability associated with the presence of hypermutability in specific genetic marker regions resulting from defective DNA mismatch repair." It can be assessed as microsatellite instability-high (MSI-H) by molecular testing or as mismatch-repair deficiency (dMMR) by testing for loss of mismatch-repair proteins. (See existing glossary entry for the full definition.)
Per NCI, MSI-H/microsatellite-unstable or dMMR status is a tissue-agnostic biomarker: NCI's PDQ on agnostic cancer therapies describes therapy for "unresectable or metastatic, microsatellite instability-high (MSI-H)/microsatellite unstable or mismatch repair-deficient (dMMR) solid tumors whose disease progressed after prior treatment," indicating doctors may test for it to help determine eligibility for certain treatments regardless of where the cancer started. This is general, source-attributed information, not medical advice.
Colorectal cancer · Endometrial cancer · Ovarian, fallopian tube, and peritoneal cancers · Head and neck squamous cell carcinoma · Non-small cell lung cancer · Small-cell lung cancer
This is general information from authoritative sources, not medical advice. What a specific test result means for your case is a question for your treating oncologist.