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second messenger

(Science: molecular biology) In many hormone sensitive systems the systemic hormone does not enter the target cell but binds to a receptor and indirectly affects the production of another molecule within the cell, this diffuses intracellularly to the target enzymes or intracellular receptor to produce the response. This intracellular mediator is called the second messenger.

Examples include cyclic AMP, cyclic GMP, iP3 and diacylglycerol.