The heterozygous IDH mutation, most commonly at an arginine (R) residue (i.e., IDH1(R132) or IHD2(R172)), results in the conversion of α‐ketoglutarate to 2‐hydroxyglutarate (2‐HG), a biomarker for IDH‐mutant gliomas. The gene discussed is IDH1; the disease is central nervous system cancer.