IDH1 and glioblastoma: According to the 2021 WHO classification of CNS tumors, former (grade 4) GBM is now classified based on the presence or absence of mutations in the isocitrate dehydrogenase (IDH) gene: IDH wild-type (IDHwt) glioblastoma or IDH-mutant (IDHmut) grade 4 astrocytoma [2].