GAST and Zollinger-Ellison syndrome: Zollinger-Ellison syndrome (ZES) has, since its original description, been characterized by extreme acid hypersecretion due to a neuroendocrine tumor ectopically secreting gastrin, resulting in severe, recalcitrant peptic ulcer disease/gastroesophageal reflux disease (GERD) that is refractory to standard anti-acid treatments.