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Molecules used to transmit signals within cells. These molecules trigger a cascade of events by activating other cellular components. The ability of cells to respond to specific extracellular molecules, or agonists, is crucial to growth, development, and homeostasis of multicellular organisms. Signal transduction refers to the movement of a signal initiated outside the cell into the cell interior. Many agonists induce the stimulation of cell growth, differentiation, or expression of specific genes. Signal transduction pathways must, therefore, include mechanisms for the initiation of signals at the cell surface membrane (plasma membrane), as well as a mechanism by which these signals traverse the interior of the cell (cytoplasm), and induce the desired target response. The pathways involve cascades of sequential molecular activation steps that are organized into three major components: (1) a receptor that recognizes and binds agonists, (2) second messengers, or signal transducing molecules, that couple receptors to intracellular pathways, and (3) effectors or molecules responsible for the ultimate response. A central feature of all signaling cascades is that they discriminate among a variety of signals and provide a mechanism for signal amplification. See Signal transduction