Permissive and instructiveHoxcodes govern limb positioning

The positioning of limbs along the anterior-posterior axis varies widely across vertebrates. The mechanisms controlling this feature remain to be fully understood. For over 30 years, it has been speculated thatHoxgenes play a key role in this process but evidence supporting this hypothesis has been largely indirect. In this study, we employed loss- and gain-of-functionHoxgene variants in chick embryos to address this issue. Using this approach, we found thatHox4/5genes are necessary but insufficient for forelimb formation. Within theHox4/5expression domain,Hox6/7genes are sufficient for reprogramming of neck lateral plate mesoderm to form an ectopic limb bud, thereby inducing forelimb formation anterior to the normal limb field. Our findings demonstrate that the forelimb program depends on the combinatorial actions of theseHoxgenes. We propose that during the evolutionary emergence of the neck,Hox4/5provide permissive cues for forelimb formation throughout the neck region, while the final position of the forelimb is determined by the instructive cues ofHox6/7in the lateral plate mesoderm.