The homeodomain protein Cdx-2 may serve as a transactivator for proglucagon and other genes. Based on the fact that many Hox and Hox-like proteins utilize Pbx1 as a co-factor and Cdx-2 possesses the penta-peptide binding motif for Pbx1, it is proposed that Pbx1 is a co-factor for Cdx-2. It is demonstrated here that Pbx1 co-transfection enhances the activation on proglucagon promoter expression by Cdx-2, and mutating the penta-peptide motif attenuates the effect of Cdx-2 on proglucagon promoter expression. Physical interaction between Cdx-2 and Pbx1 was detected by co-immunoprecipitation and GST-fusion protein pull-down assays. Interestingly, mutating or deleting the penta-peptide motif did not disrupt this interaction. Finally, my preliminary results show that Pbx1 antisense-molecule repressed Pbx1 protein expression, associated with repressed proglucagon mRNA expression. These observations support our hypothesis that Pbx1 serves as a co-factor for Cdx-2 in exerting its biological functions including activating proglucagon gene expression.