CDKN2A/B homozygous deletion is now recognized as a criterion for establishing the diagnosis of IDH-mutant astrocytoma grade 4, based on the finding of short overall survival associated with loss of both alleles.75,84–86 However, the presence of hemizygous deletion of CDKN2A/B was also shown to be a marker of less favorable outcome prognosis in patients with IDH-mutant astrocytomas when compared to patients with CDKN2A/B non-deleted tumors. This evidence concerns the gene IDH1 and astrocytoma (excluding glioblastoma).