IDH2 and astrocytoma (excluding glioblastoma): Astrocytoma IDH-mutant grade 4 (A4IDHmt) is defined as a diffuse infiltrative astrocytic glial proliferation with an IDH1 or IDH2 mutation associated with tumor necrosis, microvascular proliferation, or the homozygous deletion of the CDKN2A/B gene, or any combination of the above characteristics [3].