The fossil record suggests that fish fins suffered sequential transformations, giving rise to limbs close to the origin of tetrapods. However, the molecular mechanisms that underlie this evolutionary transition remain unclear. Interestingly, Hoxd genes exhibit a single phase of expression during zebrafish fin development but, in contrast, are expressed in two independent phases during mice limb development. The second phase is essential for digit development, the main innovation of the tetrapod limb, and is activated by enhancer elements upstream of the HoxD cluster. This suggests that acquisition of transcriptional regulators of 5’ Hoxd genes may explain the origin of digits in the tetrapod lineage (“cis” evolution hypothesis). Alternatively, these structures may have developed due to recruitment of new binding factors that acted on pre-existent 5’ Hoxd gene enhancers giving rise to a second phase of expression during limb development (“trans” evolution hypothesis). In this project, we will discern the contribution of the “cis” and the “trans” hypothesis to explain the activation of 5’ Hoxd genes that may have lead to the origin of digits in tetrapods. To this end, we will use a methodology that allows identification of highly conserved non-coding regulatory regions using Xenopus and zebrafish transgenic assays.