In the fifth edition of the “Classification of Central Nervous System Tumors,” IDHm-GBM has been redefined as “IDH-mutant astrocytoma, CNS WHO grade 4,” and GBM diagnosis is only applicable to IDHwt tumors, further establishing the central role of the IDH status in prognosis assessment.9 The gene discussed is IDH1; the disease is astrocytoma (excluding glioblastoma).