The hedgehog gene encoding a secreted protein was originally identified in Drosophila as a segment polarity gene. Three vertebrate homologs of hh have been identified; Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh). These proteins are important signaling molecules during embryonic development. Shh is expressed in embryonic tissues such as the Hensen's node, the zone of polarizing activity in the posterior limb bud, the notochord, and the floor plate of the neural tube. Hedgehog proteins are secreted as precursor molecules that undergo autocatalytic internal cleavage to yield an N-terminal domain that has signaling activity and a C-terminal domain that is active in precursor processing.