MSI-High and dMMR — What These Results Mean for Immunotherapy

Microsatellite instability (MSI) and mismatch repair deficiency (dMMR) both describe a tumour whose DNA-repair system cannot fix small copying errors that occur naturally as cells divide. Over time this lets unusually many mutations build up in the tumour's DNA. MSI is measured by PCR or NGS testing; dMMR is measured by an IHC stain for four repair proteins — both point to the same underlying biology.

This section explains what the terms generally mean. It does not interpret your personal report — please discuss your specific result with your treating oncologist.

Labs report this biomarker in one of two related formats — an MSI band from PCR/NGS testing, or an MMR band from IHC testing. The table below shows the general bands and their relevance to immunotherapy eligibility; it is educational and not an interpretation of any individual report.

IHC and NGS/PCR are two different laboratory methods that usually test the same underlying biology — your oncologist will tell you which method was used for your report and whether any confirmatory test is needed.

A tumour carrying many mutations tends to make more abnormal proteins on its surface, which the immune system can recognise as foreign. Checkpoint-inhibitor immunotherapy works by releasing a brake on immune cells so they can attack cells they recognise as abnormal — a heavily mutated MSI-High or dMMR tumour simply gives the immune system more to react to than a microsatellite-stable one.

Published data cited by NCCN and ASCO (guidance updated through 2024) put objective response rates for checkpoint-inhibitor immunotherapy in MSI-High or dMMR solid tumours roughly in the 30–40% range — meaningfully higher than in similar microsatellite-stable tumours, though the exact figure varies by cancer type, stage and how many prior treatments a patient has had. This is a response-rate range, not a survival statistic, and is not a promise of an individual outcome.

Testing is most established in colorectal and endometrial cancer, where an MSI-High/dMMR result is comparatively common, but it is used far more broadly than that today.

• Colorectal cancer — the cancer type where MSI/dMMR testing was first adopted into routine care.
• Endometrial (uterine) cancer — one of the cancers with the highest reported proportion of MSI-High/dMMR tumours.
• Gastric and small-bowel cancers — tested as part of standard molecular work-up in many centres.
• Ovarian and other solid tumours — tested when standard options are limited, or when a hereditary cause is suspected.
• Any advanced solid tumour with limited standard options — because a result here can support eligibility regardless of the organ of origin, guidelines increasingly recommend broader testing.

A result on its own does not decide anything — it starts a structured conversation. Here is the general pathway most patients go through; it is a framework for discussion with your oncologist, not a recommendation for any individual case.

1. 1

   Confirm the result and the method used

   Your oncologist confirms whether the result came from IHC (protein-based) or NGS/PCR (DNA-based) testing, coordinated through an accredited partner laboratory.

2. 2

   Full case review, not a number in isolation

   The result is reviewed alongside cancer type, stage, prior treatment and overall health — a tumour board looks at the whole picture, not one report.

3. 3

   Eligibility is discussed, never assumed

   An MSI-High or dMMR result makes immunotherapy worth discussing; it does not automatically guarantee approval, insurance coverage, or that it will be recommended.

4. 4

   A plan is agreed with you

   If immunotherapy is appropriate, your team explains the schedule, likely side effects and cost; if it isn't the right fit, standard treatment options are discussed just as thoroughly.

It can, and it's worth asking about. A dMMR or MSI-High result can arise from an inherited condition called Lynch syndrome, or it can be a sporadic change limited only to the tumour — a separate test is needed to tell the two apart. Because Lynch syndrome can affect cancer risk for blood relatives, NCCN guidance recommends discussing genetic counselling and germline testing whenever a dMMR or MSI-High result is confirmed.

This is general education, not an assessment of your family's risk. A genetic counsellor or your treating oncologist can advise whether germline testing is appropriate in your specific situation.

These companion pages go deeper into cost, coverage and other biomarkers that shape immunotherapy eligibility — worth reading alongside this one.

• Which Biomarker Tests Do You Actually Need — and What Do They Cost? — how to sequence MSI/dMMR testing with other panels without paying for tests you don't need.
• Is Biomarker Testing Covered by Insurance or Government Schemes? — what Aarogyasri, PM-JAY and private insurers typically cover for this testing.
• Tumour Mutational Burden (TMB): What a High Score Means — a related but separate biomarker sometimes tested alongside MSI/dMMR.
• Immunotherapy at CION Cancer Clinics — how biomarker results fit into the wider immunotherapy eligibility and treatment pathway.

Care Led by a Tumour Board, Not One Opinion

Every biomarker report is reviewed by a multidisciplinary team before any treatment is recommended.