An unusual type of DIABETES MELLITUS, with a strongly positive family history and which results from specific gene mutations. Persons carrying any of these mutations have a 95% risk of developing diabetes, so first-degree relatives of those diagnosed require surveillance. Management depends on which genes are involved: those with the gene termed GCK have mildly raised glucose levels, often detected by chance and are probably not at risk of any long-term complications of diabetes. Mutations with the HNF1A and HNF4A genes have a progressive defect of insulin production so that by adulthood there is insulin dependence and affected persons are susceptible to the vascular effects of diabetes.