Zollinger-Ellison syndrome (ZES) is a complex condition caused in which one or more tumors form in the pancreas. Sometimes can be found in the upper part of the small intestine (duodenum). These tumors are called gastrinomas, secrete large amounts of the hormone gastrin. Which is caused overproduction of stomach acid. The excess acid, in turn, leads to peptic of ulcers in the stomach and small bowel.