Patients with ZES have prominent gastric acid hypersecretion secondary to the hypergastrinemia with basal acid outputs (BAO) when untreated, averaging 4-fold the normal rate, reaching up to >10-fold increase in some patients [47,48], because of the trophic effects of gastrin on gastric parietal cells resulting in a 4–6-fold increase in parietal cell mass [5], which in turn results in an elevated maximal acid output (MAO) levels [47,48], as well as the BAO. The gene discussed is GAST; the disease is Zollinger-Ellison syndrome.