GAST and Zollinger-Ellison syndrome: Zollinger–Ellison syndrome (ZES) is a clinical manifestation associated with an underlying gastrinoma of the duodenum or pancreas, a neuroendocrine neoplasm (NEN) that can cause hypersecretion of gastric acid, typically leading to gastroesophageal reflux disease (GERD), recurrent peptic ulcers and chronic diarrhea due to hypersecretion of gastrin.