Molecularly defined IDH1/2-mutant astrocytomas (grades 2–4) showed the greatest heterogeneity in their original histopathologic classifications—55.0% were previously classified as astrocytomas, 24.8% as glioblastoma, 13.4% as other gliomas, and 6.8% as oligodendrogliomas. Here, IDH1 is linked to central nervous system cancer.