AbstractEhox is an X‐linked paired like homeobox gene identified from a differentiating embryonic stem (ES) cell cDNA library and is expressed at low levels in the preimplantation blastocyst and in ES cells in vitro. In embryos at 6.5 days post coitum (dpc), Ehox expression was restricted to the extraembryonic ectoderm which correlates with high‐level expression in cultures of trophoblast stem cells. Extraembryonic expression becomes further restricted to the chorion and by 15.5 dpc Ehox is expressed in chorionic trophoblast of the labyrinth and spongiotrophoblast layers of the placenta. Ehox expression in the embryo proper first appears at 8.5 dpc in the anterior foregut endoderm and by 9.5 dpc is visible in pharyngeal pouches 2–4. By 10.5 dpc, Ehox expression becomes restricted to the ventral end of pouches 2 and 3. The data presented here is the first description of Ehox expression during embryogenesis and suggests a dual role for Ehox: (1) in trophoblast stem cells and compartments of the developing placenta, and (2) during development of the pharyngeal pouches, possibly delineating the area to become thymus. Development Dynamics 228:740–744, 2003. © 2003 Wiley‐Liss, Inc.