Cholecystokinin and gastrin are two gastrointestinal peptide hormones which are structurally related. Upon de novo synthesis they can undergo post-translational modifications such as C-terminal amidation and sulfation on tyrosine residues, although non-sulfated forms also exist. Cholecystokinin and gastrin exert their acdtion through two Cholecystokinin receptors: A and B. These two receptors belong to the superfamily of G-protein coupled receptors and differently respond to sulfated and non-sulfated substrates. They localize to the central nervous system as well to peripheral tissues.