IDH1 and central nervous system cancer: Mutation of IDH1, most frequently at exon 4 codon 132 (R132H), separates low-grade gliomas from high-grade GBM and is considered a strong prognostic marker.19 This gene encodes IDH1, an enzyme that catalyzes oxidative carboxylation of isocitrate to α-ketoglutarate and is a major cellular source of cytoplasmic NADPH.