Zollinger–Ellison syndrome is caused by a gastrinoma that is a non-beta islet cell gastrin-secreting tumor of the pancreas and/or the duodenum. Gastrin secretion stimulates the acid-secreting cells of the stomach to maximal activity, with consequent gastrointestinal mucosal ulceration. It may occur sporadically or as part of an autosomal dominant familial syndrome called multiple endocrine neoplasia type 1 (MEN 1).