IDH1 is one of three isocitrate dehydrogenases that catalyze the oxidative decarboxylation of isocitrate to alpha-ketoglutarate (alpha-KG). These enzymes exist in two distinct subclasses that utilize either NAD or NADP+ respectively, as an electron acceptor (1). IDH1 is the NADP+-dependent isocitrate dehydrogenase found in the cytoplasm and peroxisomes. IDH2 and 3 are mitochodrial enzymes that also function in the Krebs cycle. IDH1 is inactivated by phosphorylation at Ser113 and contains a clasp-like domain wherein both polypeptide chains in the dimer interlock (2,3). IDH1 is expressed in a wide range of species and also in organisms that lack a complete citric acid cycle. Recently, an inactivating mutation of IDH1 has been implicated in glioblastoma (4). IDH1 appears to function as a tumor suppressor that, when mutationally inactivated, contributes to tumorigenesis in part through induction of the HIF-1 pathway (5).