The paired-homeodomain transcription factor PAX4 is expressed in the early pancreas, but is later restricted to beta cells and not expressed in mature islets, suggesting an important role of PAX4 in differentiation and development of pancreatic islet. Here we show that PAX4 mRNA was highly expressed in human insulinoma tissues, whereas little if any mRNA was expressed in normal islets. Furthermore, this insulinoma associated expression of PAX4 mRNA was accompanied with expression of its novel variant form (PAX4v). PAX4v was generated by alternative splicing lacking the exon 7, and containing intact paired and homeo domain followed by novel 35 amino acids. PAX4v reversed the wild-type PAX4 mediated repression of the insulin promoter in cotransfection assays. PAX4v may play a role to antagonize the wild-type PAX4 function in human insulinoma. These data imply a role of PAX4 and PAX4v expression in tumorigenesis and development of insulinoma.