Well differentiated duodenal NETs have been further distinguished in clinico-pathologic subtypes [162], including: gastrinomas (functioning gastrin producing NETs) which may be sporadic or MEN1/Zollinger-Ellison syndrome (ZES) associated; ampullary-type somatostatin-producing NETs which may be associated with MEN1 or Neurofibromatosis type 1; non-functioning NETs. This evidence concerns the gene GAST and Zollinger-Ellison syndrome.