Shh activates three separate pathways in this model, each with a different activation and decay threshold and rate. a. A single input motif in the hedgehog pathway is highlighted. b. Simulation reveals that the unknown "transforming" event activates immediately when the Shh signal reaches a high level. NmycP production is also activated immediately by Shh, but it rises more slowly. Gli1 production is delayed by the longer signaling chain and threshold levels of Smo and Gli2 that are required for transcription. Tumor growth requires activation of all three pathways at a sufficient level. When the Shh signal is removed, rapid decay of the transforming event and NmycP cause tumor growth to quickly stop. Gli1 levels are maintained for a short delay before declining after the Shh signal is turned off.