Mammalian Pbx genes (Pbx1-3) encode a family of TALE homeodomain proteins that function as transcriptional regulators in numerous cell types (Curr. Opin. Genet. Dev. 8 (1998) 423). The present study highlights distinctive features of Pbx1b expression during mouse embryonic development as a framework to understand its biological functions. Immunohistochemical analyses demonstrate extensive expression of Pbx1b throughout post-implantation development, with highest levels observed during early to mid-gestation. Its initial distribution is predominantly associated with condensing mesoderm, however, Pbx1b displays dynamic expression patterns in derivatives of all principal germ layers. In particular, Pbx1b localizes to sites of mesenchymal-epithelial interactions during periods of active morphogenesis in tissues such as the lung, kidney, tooth buds and vibrissae follicles. Furthermore, BrdU labeling studies reveal that Pbx1b expression domains partially overlap with regions of cellular proliferation. Taken together, these data suggest that Pbx1b contributes to multiple cellular processes during embryogenesis, which may include roles in cell-autonomous regulation as well as in the mediation of tissue interactions.