This is compounded by CDKN2A/B homozygous deletion being known to occur in only a minority of IDH-mutant astrocytomas — about 0–12% of histological grade 2 tumours and 6–20% of grade 3 [19, 20, 33, 34]. The gene discussed is CDKN2A; the disease is astrocytoma (excluding glioblastoma).