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a family of secreted intercellular signaling proteins that plays an essential role in early embryonic patterning of major organs, including the brain, eye, bone, gonads, skin, blood, gastrointestinal tract, lungs, and prostate. In mammals, three hedgehog proteins have been identified: sonic hedgehog, indian hedgehog, and desert hedgehog. Two receptors for hedgehog proteins have been identified: patched (Ptc), a negative regulator of Hh-signaling pathways, and smooth-end (Smo), a positive regulator. Abnormal functioning of the signaling pathway has been implicated in many forms of birth defects. In postembryonic life, the pathway may be activated to stimulate tissue repair or cell proliferation. Abnormal activation may lead to cancer, including basal cell carcinoma, the most common form of human cancer.