According to the 2021 WHO classification, tumors formerly known as adult-type diffuse gliomas are now categorized into three types: astrocytoma, isocitrate dehydrogenase (IDH) mutant; oligodendroglioma, IDH mutant and 1p/19q codeleted; and glioblastoma, IDH wild-type [2]. The gene discussed is IDH1; the disease is oligodendroglioma.