GCG and Zollinger-Ellison syndrome: The functioning pNETs constitute approximately 30–40% of all pNETs displaying different clinical syndromes due to hormone oversecretion by the tumor, such as excess gastrin (gastrinoma, Zollinger-Ellison syndrome), insulin (insulinoma), glucagon (glucagonoma), somatostatin (somatostatinoma) and vasoactive intestinal peptide (VIPoma).