Aird JJ, Steel MJ, Chow C, et al. - Recurrent tumors (local recurrences or metastases) from 137 people with mismatch repair (MMR) proficient primary tumors of the gastrointestinal and gynecological tracts were assessed to ascertain whether repeat testing is warranted. No cases with a genuine discordance in MMR status were found. Because MMR status does not change according to that of the primary tumor, repeat MMR IHC testing of recurrences was not found to be a necessary. Moreover, when evaluating MMR stains, the researchers of the study noted that care should be taken to avoid general pitfalls. They suggested looking for internal positive control to circumvent misinterpretation of poorly stained sections, being careful in interpreting MMR IHC in postradiotherapy samples (especially MSH6), and being attentive of the subclonal loss of MLH1 in endometrial carcinomas.