The basic helix-loop-helix (bHLH) family of transcription factors plays an important role in the normal development and function of the endocrine pancreas. Heterozygous mutations in the gene encoding one member of this family, NeuroD1/BETA2, are associated with a monogenic form of diabetes that resembles maturity-onset diabetes of the young (MODY) in many respects. This result prompted us to screen the genes encoding related bHLH transcription factors that are also expressed in pancreatic islets for diabetes-associated mutations. We have screened 57 unrelated Japanese subjects with a clinical diagnosis of MODY for mutations in the NeuroD4/Math-3/ATH-3 gene (NEUROD4). This analysis revealed seven frequent polymorphisms that were not associated with MODY, including five in the 5'-untranslated region (UTR) (-477G/A, -436delA, -324delT, -107insTTTT, and -104T/C [cDNA sequences]) and two in the 3'-UTR (1027C/T and 1076C/A). A missense mutation, K68T (203A/C), was found in a heterozygous state in one MODY subject and two nondiabetic subjects. The results of our study suggest that genetic variation in NEUROD4 is not a common cause of MODY in Japanese.