Kvon, E. Z., et. al. (2016) Cell 167:633-642 https://www.ncbi.nlm.nih.gov/pubmed/27768887
Researches have now identified a single long range limb enhancer of Sonic hedgehog responsible for normal limb development in vertebrates. By mutating the enhancer, Kvon et. al. were able to prevent normal limb development in mice and create a “Serpentized” mouse that lacks long limbs. This transcription factor is conserved throughout vertebrates including fish but is not found in snakes.