Hedgehog signalling pathway. The hedgehog gene has a role in embryo development and in the maintenance of stem cell populations in adults. Seen here are two types of transmembrane protein receptor, smoothened (red) and patched (blue). The patched receptor has a hedgehog ligand bound to it. In the absence of a bound ligand patched inhibits smoothened, preventing activation of the hedgehog signalling pathway. When a ligand is bound the inhibitory effect is lifted and the hedgehog signalling pathway is activated. This leads to the activation of the Gli transcription factor (white, centre), which travels to the nucleus (bottom centre) to promote the transcription of hedgehog targeted genes.