In mammal embryos, the Sonic hedgehog gene sets up the axis of the limb in the early stages of development. The gene encoding the secreted protein Sonic hedgehog (Shh) is expressed in the polarizing region (or zone of polarizing activity), a small group of mesenchyme cells at the posterior margin of the vertebrate limb bud. Sonic Hedgehog plays a role in cell growth, cell specialization, and the normal shaping (patterning) of the body. The gene encoding the secreted protein Sonic hedgehog (Shh) is expressed in the polarizing region (or zone of polarizing activity), a small group of mesenchyme cells at the posterior margin of the vertebrate limb bud. This lecture will therefore also introduce some concepts and experiments that have identified patterning mechanisms within the limb. Sonic Hedgehog was isolated by homology to the Hedgehog (Hh) gene. Another function of Hox genes that has emerged recently is to regulate expression of the Sonic hedgehog gene (Shh) which controls patterning of distal structures. “In a hand, for instance, Sonic hedgehog tells the limb which side will be the thumb and which side will be the pinky finger,” explains Gillis. The team concluded, as Jacob and Monod had, that one of the adjacent control regions was affected. The hedgehog signaling pathway is a mechanism that directs the development of embryonic cells in animals, from invertebrates to vertebrates. The gene encoding the secreted protein Sonic hedgehog (Shh) is expressed in the polarizing region (or zone of polarizing activity), a small group of mesenchyme cells at the posterior margin of the vertebrate limb bud. The gene encoding the secreted protein Sonic hedgehog (Shh) is expressed in the polarizing region (or zone of polarizing activity), a small group of mesenchyme cells at the posterior margin of the vertebrate limb bud. The hedgehog signaling pathway is a system of genes and gene products, mostly proteins, that convert one kind of signal into another, called transduction. SHH is a true signaling switch used in differentiating subpopulations of cells throughout the embryo. Whatever was affecting Sonic hedgehog in the limb, and causing polydactyly, wasn’t a mutation of the gene or, correspondingly, a change in its protein. TULP3 is required proper for Shh-dependent limb patterning. That area was not lit up by the dye. The reaction did not affect the gene of Sonic hedgehog. The most recent advances have been the identification of control regions that drive Hox gene expression in the limb. Thus, this molecule holds great therapeutic potential for biotechnological and biomedical approaches aiming to enhance tissue regeneration or to replace damaged tissues. A single Hh homolog is present in Drosophila, but three Hh homologs, Sonic Hh, Indian Hh, and Desert Hh, are present in mammals. The zone of polarizing activity (ZPA) is a region at the posterior margin of the limb bud that induces mirror-image duplications when grafted to the anterior of a second limb.