IDH1 and oligodendroglioma: Homozygous deletion of CDKN2A and/or CDKN2B associated with poor prognosis and IDH mutant astrocytoma; marker of WHO Grade 4 disease (Korshunov et al., 2019; Shirahata et al., 2018)Allelic losses of 9p21.3 (CDKN2A gene locus) and homozygous deletion of CDKN2A associated with poor survival in IDH mutant Grade 3 oligodendroglioma; marker of WHO Grade 3 disease (occurs in < 10% patients)