Glioblastoma is now also defined explicitly as an IDH-wildtype and histone H3-wildtype diffuse astrocytic glioma, eliminating all IDH-mutant astrocytomas regardless of their WHO grade and histologic features morphologically resembling glioblastoma, as well as eliminating H3 K27-altered diffuse midline gliomas and H3 G34-mutant diffuse hemispheric gliomas. The gene discussed is IDH2; the disease is glioblastoma.