Antennapedia

Antennapedia (abbreviated Antp) is a Hox gene first discovered in "Drosophila" which controls the formation of legs during development. Loss-of-function mutations in the regulatory region of this gene result in the development of the second leg pair into ectopic antennae. By contrast gain-of-function alleles convert antennae into ectopic legs. 
This is just one illustration of the tendency of organisms to exhibit variations on a theme: modulated repetition. Legs and antennae are related to one another as much as molars are to incisors, fingers are to toes, and arms are to legs.
"Antp" also refers to a gene complex (ANT-C) in "Drosophila" ending with the "Antp" gene. It is responsible for formation and differentiation of the thoracic and head segments of the fly's body.
Origin of "Antennapedia"-class homeobox gene.
The origin of the ancestor homeobox gene is an important aspect of the evolution of the "Antp"-class "Hox" genes. Early evolution of the "Antp"-class genes may have predated the divergence of cnidarians. However, the role that "Antp" plays in the spatial body development of cnidarians remains unclear. A widely accepted theory is that the ancestor "Hox" cluster containing three genes arose in the early metazoan era. It is suggested that Antennapedia arose from "Evx", a non-"Hox" family of genes. This duplication event of "Evx" into the "Antp"-class probably occurred prior to cnidarian divergence, as there are Cnidarians with "Evx" and without "Hox" class genes and vice versa.
"Antennapedia" in arachnids.
Recent studies have observed that down-regulation of the "Antp" gene in "Parasteatoda tepidariorum" leads to the development of a pair of ectopic legs, resulting in 10-legged mutant spiders. "Drosophila" "Antp" is thought to play an important role in the role of ectopic leg or antenna placement, but not in abdominal leg suppression. However, recent research supported that leg suppression was indeed performed by "Antp" in arachnids. This suggests that spiders and insects may have separately developed strategies of leg suppression via the evolutionary pressure of convergence. Arachnids' "Antp" gene is different from other "Antp" "Hox" clusters, suggesting that it has evolved via a divergence event leading to the development on the leg suppression function. This example suggests how the functions of homeobox genes including "Antp" have evolved over time to account for different lineages' needs.
"HoxC6": an ortholog of Antp.
Although it is known that "Antp"-class homeobox genes play some sort of role in transcriptional processes, not all of their actions and functions have been discovered. Recent studies observed "Antp" and the "Hox" ortholog "HoxC6" in "Xenopus" in order to further distinguish the evolution of these orthologues. "HoxC6" was found to play an important role in gastrulation in the vertebrate "Xenopus". However, gastrulation was also a target in the "Drosophila" "Antp" gene. The anterior–posterior pattern mechanism is highly conserved in these genes, as its function in "Xenopus" is clear, but it is unclear why it would be a target in "Drosophila". The similarities continuously observed between "Hox" genes in vertebrates and "Drosophila" suggests a complex evolutionary history of the "Antp" "Hox" gene cluster, as well as reaffirms the importance of the conservation of this gene cluster in the evolution of body morphology.