Signaling pathways that convert extracellular stimuli into intracellular responses regulate many diverse physiological and biochemical processes, and share many characteristics. An external signal binds to a cell surface receptor and stimulates a transducer to activate an internal effector, which initiates production of an intracellular messenger. The intracellular messenger sets in motion events that culminate in the overall cellular response, often by means of actions on protein kinases (Cohen, 1985 ).