Mismatch repair deficiency/microsatellite instability testing as predictive immunotherapy biomarkers—possible diagnostic missteps trusting a single method

Over the last few years, mismatch repair deficiency (MMR-D)/microsatellite instability (MSI) has been recognized as an indicator of colon carcinomas potentially susceptible to immunotherapy (1,2). To date, two strategies of identifying mismatch repair defects in tumor tissue have been employed: lack of immunohistochemical staining for mismatch repair proteins, in which case the term “mismatch repair deficiency” (MMR-D) is used, and detection of instability of microsatellite DNA tandem repeats in the tumor tissue DNA as compared to germline host DNA, in which case the term “microsatellite instability” (MSI) is used.