Aims/hypothesis. The aim of this study was to examine the prevalence and nature of mutations in HNF4α/ MODY1, GCK/MODY2 and HNF-1α/MODY3 genes in Czech subjects with clinical diagnosis of MODY. Methods. We studied 61 unrelated index probands of Czech origin (28 males, 33 females) with a clinical diagnosis of MODY and 202 family members. The mean age of probands was 22.7±12.0 years (range, 6-62) and the mean age at the first recognition of hyperglycaemia was 14.7±6.0 years (range, 1-25). The promotor and coding regions inclusive intron exon boundaries of the HNF-4α, GCK and HNF-1α genes were examined by PCR-dHPLC (HNF-1α and GCK) and direct sequencing. Results. We identified 20 different mutations in the HNF-4α, GCK and HNF-1α in 29 families (48% of all families studied), giving a relative prevalence of 5% of MODY1, 31% of MODY2 and 11.5% of MODY3 among the Czech kindred with MODY. Three of 3, 10 of 11 and 1 of 6 of the mutations identified in HNF-4α, GCK and HNF-1α respectively, were new. Conclusion/interpretation. Of the families 48% carried mutations in the MODY1-3 genes and of the identified mutations 70% were new. In 52% of Czech families with clinical characteristics of MODY, no mutations were found in the analysed genes. This finding shows that the majority of MODY mutations in a central European population are local and that other MODY genes could be responsible for autosomal dominant transmission of diabetes mellitus. [Diabetologia (2003) 46:291-295] Keywords MODY, genetics, mutation, glucokinase, HNF-1α, HNF-4α, MODY X. Corresponding author: J. Lebl, Department of Paediatrics, 3rd Faculty of Medicine, Charles University, Vinohradska 159, 100 81 Prague 10, Czech Republic E-mail: lebl@fnkv.cz Abbreviations: GCK, Glucokinase; HNF-1α, hepatocyte nuclear factor-1alpha; HNF-4α, hepatocyte nuclear factor-4alpha; IPF-1 insulin promoter factor-1; HNF-1β, hepatocyte nuclear factor-1beta; dHPLC, denatured high performance liquid chromatography; RFLP, restriction fragment length polymorphism; OHA, oral hypoglycaemic agents. Six known MODY subtypes are caused by mutations in genes encoding the hepatocyte nuclear factor-4α (HNF-4α), glucokinase (GCK), hepatocyte nuclear factor-1α (HNF-1α), insulin promoter factor-1 (IPF-1), hepatocyte nuclear factor-1β (HNF-1β) and NeuroD1 respectively [1].The relative prevalence of distinct MODY subtypes differs substantially in studies in various populations [2,3,4], mutations in GCK representing from 8 to 63% and HNF-1α mutations for 13 to 64% of all subjects with MODY [5]. Mutations in the HNF-4α, IPF-1, HNF-1β and NeuroD1 have been recognised in single families only, while additional unknown MODY genes ("MODY X") may cause between 16 and 45% of cases of MODY [3].We initiated a study of genetic epidemiology of MODY in the Czech republic, as no data on the relaMaturity Onset Diabetes of the Young is a genetically heterogeneous form of diabetes mellitus, characterised by an autosomal dominant inheritance, by early age at onset and by a primary defect in beta-cell function.