GCG and gastrin-producing neuroendocrine tumor: Among the specific markers effective in the diagnosis of certain types of GEP-NETs, which simultaneously confirm the patient’s clinical state, insulin (in insulinoma), gastrin (in gastrinoma), glucagon (in glucagonoma), vasoactive intestinal peptide (in VIPoma), and somatostatin (in somatostatinoma) should be mentioned [35].