IDH1 and oligodendroglioma: The fifth edition of the World Health Organization (WHO) Classification of Tumors of the Central Nervous System (CNS) published in 2021 classifies adult-type diffuse gliomas into IDH-mutant astrocytoma, IDH-mutant and 1p/19q-codeleted oligodendroglioma, and IDH-wildtype glioblastoma [1].