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 of prostate cancer tumours. For SHH to be expressed in the developing embryo limbs, a morphogen called fibroblast growth factors must be secreted from the apical ectodermal ridge. Sonic hedgehog has also been shown to act as an axonal guidance cue. It has been demonstrated that SHH attracts commissural axons at the ventral midline of the developing spinal cord. Specifically, SHH attracts retinal ganglion cell (RGC) axons at low concentrations and repels them at higher concentrations. The absence (non-expression) of SHH has been shown to control the growth of nascent hind limbs in cetaceans (whales and dolphins). The sonic hedgehog