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Hedgehog signalling. (A) Hedgehog ligands (Hhl) bind to PTCH1 and unrepress SMO with activation of GLI and target genes. (B) The tumor produces Hhl and stimulates itself. (C) Tumor cells produce Hhl and activate signaling in nonmalignant cells. In turn, other signaling pathways are activated and stimulate tumor growth (arrow). (D) Stromal cells produce the Hhl required for tumor growth/survival.
