Later studies demonstrated that the acid hypersecretion was secondary to the ectopic release of gastrin by a neuroendocrine tumor (i.e., gastrinoma) [6,7,8], the gastrinomas were more frequently located in the duodenum than in the pancreas [4,9,10,11,12,13,14], and that 20–25% of all patients had ZES as part of the autosomal dominant disorder, Multiple Endocrine Neoplasia-type 1 (MEN1), which is characterized by multiple endocrine tumors/hyperplasia of the pancreas and the parathyroid and pituitary glands [15,16,17,18,19]. This evidence concerns the gene GAST and gastrin-producing neuroendocrine tumor.