Proteins that bind cyclic nucleotides (cAMP or cGMP) share a structural domain of about 120 residues [<cite idref="PUB00013967"/>,<cite idref="PUB00013968"/>, <cite idref="PUB00005543"/>]. The best studied of these proteins is the prokaryotic catabolite gene activator (alsoknown as the cAMP receptor protein) (gene crp) where such a domain is known to be composed of three alpha-helices anda distinctive eight-stranded, antiparallel beta-barrel structure. There are six invariant amino acids in this domain,three of which are glycine residues that are thought to be essential for maintenance of the structural integrity ofthe beta-barrel. cAMP- and cGMP-dependent protein kinases (cAPK and cGPK) contain two tandem copies of the cyclicnucleotide-binding domain. The cAPK's are composed of two different subunits, a catalytic chain and a regulatory chain,which contains both copies of the domain. The cGPK's are single chain enzymes that include the two copies of the domainin their N-terminal section. Vertebrate cyclic nucleotide-gated ion-channels also contain this domain. Two suchcations channels have been fully characterised, one is found in rod cells where it plays a role in visual signaltransduction.