Our study strongly suggests that in IDH-mutant gliomas, ATRX loss indicates astrocytic phenotype, ATRX retention, and H3K27me3 loss indicate oligodendroglial phenotype, while retained or non-conclusive ATRX and retained H3K27me3 mandate for 1p/19q codeletion testing. This evidence concerns the gene IDH1 and glioma.