Transcription of the ITGB3 gene, which encodes beta3 integrin, increases during myeloid differentiation. alphavbeta3 integrin mediates adhesion to fibronectin or vitronectin and regulates various aspects of the inflammatory response in mature phagocytes. In these studies, we found that the homeodomain transcription factor HoxA10 interacted with a specific ITGB3 cis element and activated transcription of this gene during myeloid differentiation. We also found that increased fibronectin adhesion in differentiating myeloid cells was dependent upon this HoxA10-induced increase in beta3 integrin expression. We determined that activation of ITGB3 transcription required a HoxA10 domain that was not identical to the "hexapeptide" that mediates interaction of Hox and Pbx proteins. This activation domain was also not identical to a previously identified HoxA10 repression domain that mediates interaction with transcriptional co-repressors. Instead, this HoxA10 activation domain had homology to "PQ" protein-protein interaction domains that have been described previously in other transcription factors. Consistent with this, we found that the HoxA10 PQ-like domain recruited the CREB-binding protein (CBP) to the ITGB3 promoter. This was associated with an increase in local histone acetylation in vivo. In immature myeloid cells, we previously determined that HoxA10 repressed transcription of the CYBB and NCF2 genes, which encode the phagocyte oxidase proteins gp91(PHOX) and p67(PHOX), respectively. Therefore, our studies indicated that HoxA10 either activates or represses gene transcription at various points during myelopoiesis. Our studies also suggested that HoxA10 is a bifunctional protein that is involved in dynamic regulation of multiple aspects of phagocyte phenotype and function.