Increasing evidence suggests that defects in genes encoding transcription factors that are expressed in the pancreatic beta-cells may be important contributors to the genetic basis of type 2 diabetes mellitus. Maturity-onset diabetes of the young (MODY) now exists in five subtypes (MODY1-5), four of which are caused by mutations in transcription factors hepatocyte nuclear factor-4alpha (HNF-4alpha), HNF-1alpha, insulin promoter factor-1 (IPF-1), and HNF-1beta (MODY1, -3, -4, and -5). Recent evidence from the British population even suggested that IPF-1 may be a predisposing gene for type 2 diabetes. Thus, highlighting the potential role of this transcription factor in the genetic basis of Danish and Italian MODY as well as in Danish patients with late-onset type 2 diabetes mellitus, we have examined the human IPF-1 gene for mutations by single strand conformation polymorphism and heteroduplex analysis in 200 Danish patients with late-onset type 2 diabetes and in 44 Danish and Italian MODY patients. In the patients with late-onset type 2 diabetes we identified a noncoding G insertion/deletion polymorphism at nucleotide -108, a silent G54G, and a rare missense D76N variant. Moreover, a Danish MODY patient was carrier of an A140T variant. Neither the D76N nor the A140T segregated with diabetes, and their transcriptional activation of the human insulin promoter expressed in vitro was indistinguishable from that of the wild type (115 +/- 21% and 84 +/- 12% vs. 100%). We conclude that variants in IPF-1 are not a common cause of MODY or late-onset type 2 diabetes in the Caucasian population, and that in terms of insulin transcription both the N76 and the T140 mutations are likely to represent functionally normal IPF-1 variants with no direct role in the pathogenesis of MODY or late-onset type 2 diabetes mellitus.