Hox genes provide segmental identity during early vertebrate development but their role later in development is unclear. On , Santagati et al. describe a temporal requirement for Hoxa2 during the patterning of the branchial arches, structures that are generated by neural crest cells(NCCs) in the mouse head. The authors report that conditionally inactivating Hoxa2 at various time points in mouse development converts second branchial arch elements (the hyoid skeleton, which lies at the base of the tongue) into first arch elements (jaw and middle ear structures). Their findings show that hyoid NCCs retain considerable plasticity long after their migration into the second arch, and that Hoxa2 function controls their morphogenesis, acting at separate time points to pattern distinct second arch derivatives. This temporal analysis of Hox gene function in a vertebrate embryo provides insights into Hox requirements in late morphogenetic processes.
