Biomarkers
Microsatellite Instability-High
Also known as: MSI-H, dMMR, Mismatch repair deficiency
Definition
A genetic condition where DNA mismatch repair is defective, leading to high mutation rates and immunotherapy responsiveness.
Detailed Explanation
MSI-H (also called dMMR for deficient mismatch repair) occurs in ~15% of colorectal cancers and some endometrial, gastric, and other cancers. It can be hereditary (Lynch syndrome) or sporadic. MSI-H tumors have high tumor mutational burden and respond very well to checkpoint inhibitors like pembrolizumab, with response rates of 40-60%.