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]. The gene discussed is IDH1; the disease is astrocytoma (excluding glioblastoma).