IDH2 and astrocytoma (excluding glioblastoma): CDKN2A/B HD has been demonstrated to be strongly associated with poor prognosis in IDH-mutant astrocytomas [37], and the WHO CNS5 defines IDH-mutant astrocytomas with CDKN2A/B HD as CNS WHO grade 4, regardless of histological findings [8, 15, 22].