IDH-mutant diffuse gliomas represent biologically distinct neoplasms compared to the IDH-wildtype glioblastoma and, for this reason, the WHO 2021 classification restricted the use of the glioblastoma term to this entity, replacing the previous IDH-mutant glioblastoma with the novel astrocytoma, IDH-mutant, grade 4. The gene discussed is IDH1; the disease is glioblastoma.