RAR beta mediates the response of Hoxd4 and Hoxb4 to exogenous retinoic acid

One action of retinoids in development is the regulation of Hox gene expres sion. Hoxd4 and Hoxb4 expression in the embryonic hindbrain is anteriorized by retinoic acid (RA) treatment of mid-gestation mouse embryos. Here we de monstrate that retinoic acid receptor beta (Rarb) deficient mice present on ly a partial anteriorization of either Hoxd4 or Hoxb4 in response to RA tre atment. Our results strongly suggest that RAR beta is a mediator of their R A-response, and reveal anteroposterior polarity within a single rhombomere (r). Additionally, we generated mice doubly mutated for Hoxd4 and Rarb in a n attempt to identify common morphogenetic pathways between these two genes . We conclude that there are no synergistic interactions between Hoxd4 and Rarb in the specification of the cervical vertebrae. Dev Dyn 1999;215:96-10 7. (C) 1999 Wiley-Liss, Inc.