MSI/dMMR in Colorectal Cancer: Immunotherapy Eligibility

A biomarker that unlocks immunotherapy

Deficient mismatch repair (dMMR) or microsatellite instability-high (MSI-H) colorectal tumors are hypermutated and respond to immune checkpoint inhibitors, even in metastatic disease.

Testing and impact

• dMMR/MSI-H status is checked by immunohistochemistry or PCR.
• Positive status can make immunotherapy a front-line option regardless of site.
• It also carries prognostic and familial-syndrome implications.

Staging context

Biomarker status is recorded alongside TNM because it can shift the systemic plan more than stage alone in some cases.