Adult-type diffuse gliomas are grouped as those with isocitrate dehydrogenase (IDH) mutations (IDH-mutant) and those without IDH mutations (IDH-wildtype), and they constitute the three categories of IDH-mutant astrocytoma (grade 2, 3, or 4), IDH-mutant and 1p/19q-codeleted oligodendroglioma (grade 2 or 3), and IDH-wildtype glioblastoma (GB; grade 4). This evidence concerns the gene IDH1 and oligodendroglioma.