Until relatively recently, the consensus has been that heterozygous highly penetrant loss-of-function alleles in HNF1A give rise to a clinically distinct diabetes subtype, characterized by an early age of onset (typically < 25 years), dominant inheritance, sensitivity to sulphonylureas, and non-obesity, and termed HNF1A maturity-onset diabetes of the young (HNF1A-MODY [MIM: 600496]).1 The gene discussed is HNF1A; the disease is diabetes mellitus.