IDH1 and oligodendroglioma: Diffuse gliomas are the most frequent adult-onset primary intra-axial malignancies1 and are classified in the WHO 2016 criteria according to histologic and molecular–genetic characteristics as grade 2, 3, or 4.2 High-grade gliomas (HGGs, WHO grade 3/4) include isocitrate dehydrogenase (IDH)–mutant astrocytomas, 1p/19q-codeleted oligodendrogliomas, and the more aggressive IDH–wild-type (WT) tumors.