The grading of IDH-mutant astrocytomas also relies on a combination of histomorphological, as well as molecular traits: the presence of CDKN2A/B homozygous deletion and/or necrosis and/or microvascular proliferation defines a CNS WHO grade 4 lesion in IDH-mutant astrocytomas.2 The gene discussed is IDH2; the disease is astrocytoma (excluding glioblastoma).