Maturity-onset diabetes of the young (MODY) refers to any of several hereditary forms of diabetes mellitus caused by mutations in an autosomal dominant gene disrupting insulin production. Along with neonatal diabetes, MODY is a form of the conditions known as monogenic diabetes. While the more common types of diabetes (especially type 1 and type 2) involve more complex combinations of causes involving multiple genes and environmental factors, each forms of MODY are caused by changes to a single gene (monogenic). GCK-MODY (MODY 2) and HNF1A-MODY (MODY 3) are the most common forms. Robert Tattersall and Stefan Fajans initially identified the phenomenon known as maturity onset diabetes of the young in a classic study published in the journal Diabetes in 1975. Signs and symptoms MODY is the final diagnosis in 1%–2% of people initially diagnosed with diabetes. The prevalence is 70–110 per million people. 50% of first-degree relatives will inherit the same mutation, giving them a greater than 95% lifetime risk of developing MODY themselves. For this reason, correct diagnosis of this condition is important. Typically patients present with a strong family history of diabetes (any type) and the onset of symptoms is in the second to fifth decade. There are two general types of clinical presentation. Some forms of MODY produce significant hyperglycemia and the typical signs and symptoms of diabetes: increased thirst and urination (polydipsia and polyuria). In contrast, many people with MODY have no signs or symptoms and are diagnosed either by accident, when a high glucose is discovered during testing for other reasons, or screening of relatives of a person discovered to have diabetes. Discovery of mild hyperglycemia during a routine glucose tolerance test for pregnancy is particularly characteristic. MODY cases may make up as many as 5% of presumed type 1 and type 2 diabetes cases in a large clinic population. While the goals of diabetes management are the same no matter what type, there are two primary advantages of confirming a diagnosis of MODY. Insulin may not be necessary and it may be possible to switch a person from insulin injections to oral agents without loss of glycemic control. It may prompt screening of relatives and so help identify other cases in family members. As it occurs infrequently, many cases of MODY are initially assumed to be more common forms of diabetes: type 1 if the patient is young and not overweight, type 2 if