GAST and gastroesophageal reflux disease: Purpose: Zollinger–Ellison syndrome (ZES) is the most frequent, functional, malignant pancreatic neuroendocrine tumor syndrome (pNET), which is due to ectopic secretion of gastrin by a pNET/NET (i.e., gastrinomas) resulting in severe, refractory acid-peptic disease (ulcer, GERD).