THE disease non-insulin-dependent (type 2) diabetes mellitus (NIDDM) is characterized by abnormally high blood glucose resulting from a relative deficiency of insulin¹. It affects about 2% of the world's population and treatment of diabetes and its complications are an increasing health-care burden². Genetic factors are important in the aetiology of NIDDM, and linkage studies are starting to localize some of the genes that influence the development of this disorder³. Maturity-onset diabetes of the young (MODY), a single-gene disorder responsible for 2–5% of NIDDM, is characterized by autosomal dominant inheritance and an age of onset of 25 years or younger4–6. MODY genes have been localized to chromosomes 7, 12 and 20 (refs 5, 7, 8) and clinical studies indicate that mutations in these genes are associated with abnormal patterns of glucose-stimulated insulin secretion^1,9. The gene on chromosome 7 (MODY2) encodes the glycolytic enzyme glucokinase⁵ which plays a key role in generating the metabolic signal for insulin secretion and in integrating hepatic glucose uptake. Here we show that subjects with the MODY3-form of NIDDM have mutations in the gene encoding hepatocyte nuclear factor-1α (HNF-1α, which is encoded by the gene TCF1). HNF-1α is a transcription factor that helps in the tissue-specific regulation of the expression of several liver genes^10,11 and also functions as a weak transactivator of the rat insulin-I gene¹².