A derivative of ATP that is widespread in cells as a second messenger in many biochemical reactions induced by hormones. Binding of the hormone to its receptor on the cell surface activates G proteins, which in turn activate (or in some cases inhibit) adenylate cyclase, the enzyme that catalyses the formation of cAMP. cAMP removes a regulatory subunit from the enzyme protein kinase A, thereby enabling it to phosphorylate (and hence activate) intracellular proteins that mediate the ultimate effects of the hormone on the cell. The stimulatory signal wanes as cAMP is degraded to AMP by a phosphodiesterase, and phosphorylated proteins are dephosphorylated and inactivated. Cyclic AMP is also involved in controlling gene expression and cell division, in immune responses, and in nervous transmission.