Dong F, et al. - Molecular features of mismatch repair deficiency, explicitly insertion and deletion mutations in mononucleotide repeats, were investigated using a targeted next-generation sequencing assay. In addition, determination of the number of such mutations needed to classify endometrial cancers as mismatch repair deficient, proficient, or indeterminate was performed. Further, using immunohistochemistry, a comparison of sequencing classification to the loss of MLH1, MSH2, MSH6, or PMS2 expression was done. Findings revealed a high rate of concordance between targeted next-generation sequencing and immunohistochemistry for mismatch repair deficiency, but sequencing was indeterminate in a few cases and demonstrated a false negative rate of 5%. Researchers recommended implementation of a mismatch repair deficiency algorithm for laboratories performing next-generation sequencing cancer panels, for mismatch repair deficiency in endometrial cancer, immunohistochemistry is a cost-effective screening method.