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Homology of process in the formation of the anterior-posterior axes in Drosophila and chick appendages. A chick limb bud expresses Sonic hedgehog in its posterior region. If Sonic hedgehog is also expressed in an anterior region, the limb develops a mirror-image duplication of the anterior-posterior axis. A Drosophila wing disc expresses Hedgehog in its posterior compartment. If Hedgehog is expressed in the anterior compartment as well, the wing develops a mirror-image duplication of the anterior-posterior axis. (After Ingham 1994.)