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Background: microsatellite instability (MSI) is commonly screened using a panel of two mononucleotide and three dinucleotide repeats as recommended by a consensus meeting on MSI tumours held at the National Cancer Institute (Bethesda, MD, USA). According to these recommendations, tumours are classified as MSI-H when at least two of the five microsatellite markers show instability, MSI-L when only one marker shows instability and MSS when none of the markers show instability. Almost all MSI-H tumours are characterised by alterations in one of the four major proteins of the mismatch repair (MMR) system (MLH1, MSH2, MSH6 or PMS2) that renders them MMR deficient, whereas MSI-L and MSS tumours are generally MMR proficient. However, tumours from patients with a pathogenic germline mutation in MSH6 can sometimes present an MSI-L phenotype with the NCI panel. The MSH6 protein is not involved in the repair of mismatches of two nucleotides in length and consequently the three dinucleotide repeats of the NCI panel often show stability in MSH6-deficient tumours.Methods: a pentaplex panel comprising five mononucleotide repeats has been recommended as an alternative to the NCI panel to determine tumour MSI status. Several studies have confirmed the sensitivity, specificity and ease of use of the pentaplex panel; however, its sensitivity for the detection of MSH6-deficient tumours is so far unknown. Here, we used the pentaplex panel to evaluate MSI status in 29 tumours known to harbour an MSH6 defect.Results: MSI-H status was confirmed in 15 out of 15 (100%) cases where matching normal DNA was available and in 28 out of 29 (97%) cases where matching DNA was not available or was not analysed.Conclusion: these results show that the pentaplex assay efficiently discriminates the MSI status of tumours with an MSH6 defect. [ABSTRACT FROM AUTHOR]