Gastrin and cholecystokinin (CCK) [1,2] are structurally and functionally related peptide hormones that function as hormonal regulators of various digestive processes and feeding behaviors . They are known to induce gastric secretion, stimulate pancreatic secretion, increase blood circulation and water secretion in the stomach and intestine, and stimulate smooth muscle contraction. Originally found in the gut, these hormones have since been shown to be present in various parts of the nervous system. Like many other active peptides they are synthesized as larger protein precursors that are enzymatically converted to their mature forms. They are found in several molecular forms due to tissue-specific post-translational processing. A number of other peptides are known to belong to the same family:

• Caerulein [3], an amphibian skin peptide, with a biological activity similar to that of CCK or gastrin. There are different types of caerulein precursors [4] in which a single or up to four copies of the peptide are present.
• Leukosulfakinin I and II (LSK) [5,6] are peptides, isolated from cockroach, that change the frequency and amplitude of contractions of the hindgut.
• Drosulfakinins I and II [7] are putative CCK-homologs from Drosophila. Those two peptides are part of a precursor sequence that was isolated using a probe based on the sequence of CCK and LSK.
• A chicken antrum peptide [8] which is a potent stimulus of avian gastric acid but not of pancreatic secretion.
• Cionin [9], a neuropeptide from the protochordate Ciona intestinalis.

The biological activity of gastrin and CCK is associated with the last five C-terminal residues. One or two positions downstream, there is a conserved sulfated tyrosine residue. The signature pattern developed for this family of peptides includes the biologically active C-terminal sequence as well as the sulfated tyrosine.