MRC1 and cancer: For those institutions where such testing is unavailable, paired testing with PCR-based MSI may be a useful adjunct to MMR IHC when the underlying etiology of IHC loss is unclear or atypical, although MSI PCR may have limitations in noncolorectal cancers.35 In the setting of a benign germline variant that causes false loss on MMR IHC testing, the expected pattern would be isolated loss of an MMR protein expression and discordant MSS status.