Whole-arm chromosomal losses of chromosomes 1p and 19q differentiate IDH mutant oligodendroglioma from IDH mutant astrocytoma (Brown et al., 2021; Woehrer and Hainfellner, 2015)1q and 19p co-polysomy concurrent with 1p/19q codeletion associated with earlier recurrence and shorter survival in IDH mutant oligodendroglioma (Chen et al., 2019; Snuderl et al., 2009). The gene discussed is IDH1; the disease is astrocytoma (excluding glioblastoma).