Cyclic guanosine monophosphate: a derivative of the nucleotide guanosine triphosphate (GTP) that, like cyclic AMP, acts as a second messenger in signalling pathways within cells. Formed from GTP by the enzyme guanylate cyclase, cyclic GMP activates protein kinase G, which in turn activates specific intracellular proteins by phosphorylation. Cyclic GMP is found in most animal cells; one of its key roles is in rod cells of the retina, where it regulates light-dependent opening and closing of ion channels in the rod-cell plasma membrane (see transducin).