Gastrin is a peptide hormone that is important in the regulation of gastric acid secretion and mucosal cell proliferation. Progastrin is synthesized in G cells in the antropyloric mucosa, extra antral G cells and in endocrine cells of the pancreas and the pituitary gland. In G cells the progastrin is concentrated into secretory granules, cleaved in the N- and C-terminal and amidated to form gastrin. The two major forms of gastrin are gastrin-34 and gastrin-17. Secreted gastrin acts by binding to the G-coupled CKK2 receptor and stimulates the release of histamine from enterochromaffin-like cells. The released histamine acts as a paracrine stimulant on parietal cells that release gastric acid into the gut lumen. Gastrinomas are gastrin secreting neuroendocrine tumors associated with the Zollinger-Ellison (ZES) syndrome or with the autosomal dominant inherited disorder multiple endocrine neoplasia type 1 (MEN1) syndrome. Sporadic gastrinomas are generally single and most commonly found in the pancreas but can be located to the duodenum. MEN1 associated gastrinomas are most often multiple and located to the duodenum.