IDH1 and astrocytoma (excluding glioblastoma): Similar to IDH-mutant astrocytomas, homozygous deletion of CDKN2A/B in oligodendrogliomas is linked to inferior clinical outcomes, even in cases without microvascular proliferation and/or necrosis, and WHO CNS5 suggests that this may be used as a molecular marker of grade 3 tumours in cases that have borderline histology [34].