Contributions of the Zollinger-Ellison syndrome. The salient contributions of the Zollinger-Ellison syndrome have made it unique. No pancreatic endocrine tumor described before (insulinoma) or subsequently (glucagonoma, somatostatinoma, vipoma, pancreatic-polypeptidoma) has been the topic of such a variety of studies, or has been such an inspiration and rich source of new ideas for investigation and ultimate improvement in patient care. 