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Hedgehog signaling. Paracrine Hedgehog signaling occurs when ligand (depicted here as Sonic Hedgehog, Shh) is secreted. Shh then binds to the Patched receptor (Ptch) on the target cell. When unbound by ligand, Ptch inhibits the transmembrane protein Smoothened (Smo). However, once ligand binds to Ptch, Smo is relieved of inhibition, leading to the activation of the Gli family of transcription factors and target gene expression.
