What should you know about MLH1 immunohistochemistry?

A pathologist applies antibodies to preserved tissue, often from a biopsy or surgery specimen. The stain is examined under a microscope and compared with control tissue. The report may describe MLH1 staining as retained or lost, sometimes alongside other mismatch-repair proteins such as PMS2, MSH2 and MSH6.

Loss of MLH1 staining suggests that the mismatch-repair pathway may not be working normally in the tested tumour. It is not, by itself, a diagnosis of an inherited condition or a complete treatment decision. The result is interpreted with the tumour type, medical history and other laboratory findings.

Ask your team to explain the exact wording of the report and whether the internal controls were valid. If MLH1 is absent, additional testing may help distinguish a tumour change acquired in the cancer from a possible inherited predisposition. A genetics referral may be considered when personal or family history supports it.

Keep copies of the pathology report and biopsy details. Treatment planning may use this result with imaging, stage and other biomarkers. Do not interpret a stain result alone as proof that relatives have a condition or that a particular medicine is required.

The stain itself does not cause symptoms and usually does not require urgent action. Contact your care team promptly if you develop fever, worsening pain, bleeding from a biopsy site, repeated vomiting or sudden weakness after a procedure. Seek emergency help for heavy bleeding, breathing difficulty, fainting or severe allergic symptoms.

Was MLH1 retained or lost, and were the control stains valid? Were the other mismatch-repair proteins tested? Is further tumour testing or genetic counselling appropriate? How does this result fit with my cancer stage and treatment options? Should my relatives discuss their family history with a clinician?