# What is MSI-high / dMMR?

Source: https://oncologyistanbul.com/glossary/msi-dmmr

> Lab findings — microsatellite instability-high (MSI-H) or mismatch-repair-deficient (dMMR) — that identify tumors particularly likely to respond to immunotherapy.

Tumors with a faulty DNA mismatch-repair system (dMMR) accumulate a distinctive pattern of genetic errors called microsatellite instability (MSI-H). Testing for this — common in colorectal and some other cancers — matters because MSI-H/dMMR tumors respond to checkpoint-inhibitor immunotherapy meaningfully more often than tumors without this feature, making the test a standard part of the initial workup for colorectal cancer.

## Related
- [Colorectal cancer treatment](https://oncologyistanbul.com/cancers/gi)
- [Immunotherapy](https://oncologyistanbul.com/treatments/immunotherapy)

Full glossary: https://oncologyistanbul.com/glossary