IDH1 and oligodendroglioma: According to the up-to-date WHO classification of CNS tumors, diffuse gliomas in adults have been classified into: astrocytoma IDH-mutant (grade 2, 3, or 4), oligodendroglioma IDH-mutant and 1p/19q co-deleted (grade 2 or 3), and glioblastoma (GBM) IDH-wildtype (grade 4) (4).