Maturity-onset diabetes of the young (MODY) is a genetically and clinically heterogeneous subtype of Type 2 diabetes characterized by early onset, autosomal dominant inheritance and primary defects in insulin secretion. To date, five proteins have been identified whose genetic absence or impairment causes MODY, the enzyme glucokinase (GCK/MODY2) and four transcription factors: hepatocyte nuclear factor 4α (HNF-4α/MODY1), HNF-1α/MODY3, insulin promoter factor 1 (IPF-1/MODY4) and HNF-1β/MODY5. Additional MODY genes remain to be identified.