The Nkx homeodomain proteins are members of a growing family of known vertebrate transcription factors that are believed to play a role in cell type specification and/or maintenance of the differentiated phenotype. In this article we report on the identification and developmental expression pattern of the mouse Nkx2-3 gene. The gene is expressed primarily in gut mesoderm, dinstinct regions of the branchial arches, the tongue epithelium, and limited domains in the developing jaws, possibly including tooth anlagen. In contrast to the chicken and Xenopus genes, Nkx2-3 expression in the mouse was not observed in the developing heart or neural tube. Thus, although structurally related to chicken and Xenopus Nkx2-3, the mouse gene exhibits an overlapping but distinct expression pattern that may suggest the existence of additional family members, as yet unidentified, in vertebrate organisms.