VIP and gastrin-producing neuroendocrine tumor: Based on the secreted peptide, the functioning PanNENs can be divided into insulinoma (insulin), gastrinoma (gastrin), glucagonoma (glucagon), somatostatinoma (somatotastin) and VIPoma (vasoactive intestinal peptide; VIP), along with other less common tumors producing serotonin and adrenocorticotrophic hormone (ACTH) [5,6,7].