Antennapedia

thumb|right|Fly with legs in place of antennae, caused by Antennapedia mutation thumb|right|The two Hox gene complexes in fruit flies 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.