In this study, we analyzed 18 IDH-mutant astrocytomas and 20 IDH-wild-type glioblastomas with MMR mutations at initial presentation, compared to 361 IDH-mutant astrocytomas and 906 IDH-wild-type glioblastomas without MMR mutations from institutional and publicly available cohorts of diffuse gliomas to evaluate the clinical, pathologic, and molecular features, as well as the prognostic impact of MMR mutations in these tumors. The gene discussed is MRC1; the disease is glioblastoma.