In the 2021 WHO classification, adult-type diffuse gliomas are subclassified as astrocytoma, IDH-mutant (grade 2, 3, or 4); oligodendroglioma, IDH-mutant and 1p/19q-codeleted (grade 2 or 3); molecular glioblastoma (lower grade astrocytoma with chromosome 7 gains/chromosome 10 losses, epidermal growth factor receptor (EGFR) amplification, and/or telomerase reverse transcriptase (TERT) and glioblastoma, IDH-wildtype (grade 4). The gene discussed is IDH1; the disease is oligodendroglioma.