Zollinger-Ellison syndrome (ZES) is a rare condition characterized by the development of one or more tumors, called gastrinomas, in the pancreas or duodenum. These tumors cause excessive production of gastrin, a hormone that stimulates the stomach to produce large amounts of gastric acid, leading to severe and recurrent peptic ulcers, gastroesophageal reflux disease (GERD), and diarrhea.