Maturity-onset diabetes of the young (MODY) is an autosomal dominant-subtype of noninsulin-dependent diabetes mellitus, characterized by an early age of onset. MODY is considered a model for genetic studies of noninsulin-dependent diabetes mellitus because of the availability of large multigenerational families, which make linkage analyses possible. So far, two MODY susceptibility loci have been reported, one unknown gene on chromosome 20q, and glucokinase on chromosome 7q. Glucokinase mutations appear to be the most common cause of MODY in the European population, being found in 60% of the families investigated. This form of diabetes results from a primary defect in insulin secretion due to the reduced enzymatic activity of the mutant glucokinase. The search for a third MODY gene will provide a better understanding of diabetes in childhood.