Recurrent somatic mutations in the isocitrate dehydrogenase 1 (IDH1) and IDH2 genes that result in the accumulation of D-2-hydroxyglutarate (D-2-HG) have been identified in malignant gliomas and in acute myeloid leukemia (AML). However, the function of this metabolite in normal and malignant tissues remains uncertain. A report in the current issue of Science describes a germline IDH2 mutation in a subset of patients with a rare metabolic disorder--D-2-hydroxyglutaric aciduria-that is similar to mutations seen in cancer patients. These observations further elucidate the effects of IDH mutations on normal and malignant cells.