Sonic the Hedgehog protagonist Sonic the Hedgehog, a blue hedgehog able to run at supersonic speeds, and recurring villain Dr. Eggman, an evil scientist whose ambition is to conquer the world. Among Sonic's many allies, the most well-known include: Miles "Tails" Prower, a twin-tailed, highly intelligent yellow fox who is Sonic's sidekick and best friend; Amy Rose, a pink hedgehog who is Sonic's self-proclaimed girlfriend and a hero in her own right; Knuckles the Echidna, a coolheaded red echidna who is the guardian of the Master Emerald; Shadow the Hedgehog, an anti-heroic doppelgänger of Sonic with a dark past; and Metal Sonic, a
Sonic hedgehog including the limb and midline structures in the brain, spinal cord, the thalamus by the "zona limitans intrathalamica" the lungs, and the teeth. Mutations in the human sonic hedgehog gene, "SHH", cause holoprosencephaly type 3 HPE3 as a result of the loss of the ventral midline. Sonic hedgehog is secreted at the zone of polarizing activity, which is located on the posterior side of a limb bud in an embryo. The sonic hedgehog transcription pathway has also been linked to the formation of specific kinds of cancerous tumors, including the embryonic cerebellar tumor, and medulloblastoma, as well as the progression