Functional tumors secrete bioactive substances such as insulin, glucagon, vasoactive intestinal peptide (VIP), and gastrin in cases of pancreatic NETs (pNETs) or 5-hydroxyindoleacetic acid (5-HIAA) in cases of small bowel NETs, which can lead to clinical syndromes, such as the carcinoid syndrome [8]. Here, VIP is linked to carcinoid syndrome.