Nearly all 1p/19q codeleted gliomas (oligodendroglial tumors) harbor an IDH1/2 mutation, along with O6-methylguanine-DNA methyltransferase (MGMT) promoter methylation and telomerase reverse transcriptase (TERT) promoter mutation, while TP53 and ATRX alterations have been found in IDH1/2-mutated glioma without codeletion (astrocytic tumors). The gene discussed is IDH1; the disease is oligodendroglial tumor.