by Jason Wasserman MD PhD FRCPC
April 18, 2022

What are mismatch repair proteins?

Mismatch repair proteins (MMR) are a system inside all normal, healthy cells for fixing mistakes in our genetic material (DNA). The system is made up of different proteins and the four most common are called MSH2, MSH6, MLH1, and PMS2.

The four MMR proteins MSH2, MSH6, MLH1, and PMS2 work in pairs to fix damaged DNA. Specifically, MSH2 works with MSH6 and MLH1 works with PMS2. If one protein is lost, the pair cannot function normally. A loss of one of these proteins increases the risk of developing cancer.

Pathologists order MMR testing to see if any of these proteins are lost in a tumour. If MMR testing has been ordered on your tissue sample, the results will be described in your pathology report.

How do pathologists test for mismatch repair proteins?

The most common way to test for MMR proteins is to perform a test called immunohistochemistry. This test allows pathologists to see if the tumour cells are producing all four mismatch repair proteins.

If the tumour cells are not producing one of the proteins, your report will describe this protein as “lost” or “deficient”. Because the MMR proteins work in pairs (MSH2 + MSH6 and MLH1 + PMS2), two proteins are often lost at the same time.

If the tumour cells in your tissue sample show a loss of one or more MMR proteins, you should be referred to a genetic specialist for additional tests and advice.