GAST and Zollinger-Ellison syndrome: Most common functional pNENs include insulinomas, characterized by severe hypoglycemia with a usually low risk of malignant behavior (5% of cases), and gastrinomas caused by hypersecretion of gastrin which is responsible for a clinical syndrome known as Zollinger-Ellison syndrome (ZES) characterized by gastroesophageal reflux disease (GERD), peptic ulcer disease (PUD), and diarrhea with a good response to proton pomp inhibitors (PPI) therapy [3, 9–11].