IDH1 and glioblastoma: According to the 2021 World Health Organization (WHO) Classification of Tumors of the Central Nervous System, glioblastoma is defined as an IDH-wildtype astrocytic tumor, CNS WHO grade 4, whereas astrocytic tumors that arise from lower-grade precursors and harbor IDH mutations are now classified as astrocytoma, IDH-mutant, CNS WHO grade 4.