Maturity onset diabetes of the young (MODY) is a subtype of familial diabetes mellitus characterised by early onset, autosomal dominant inheritance and primary defects of insulin secretion. Mutations in six known genes (the enzyme glucokinase and five transcription factors expressed in pancreatic beta-cells) cause most of the MODY cases. This genetic heterogeneity is associated with metabolic and clinical heterogeneity making MODY an interesting model of genotype/phenotype interaction in diabetes.