GCG and pancreatic neuroendocrine tumor: However, the positivity at immunohistochemical staining for serotonin (57% of patients), CDX-2 (60% of patients) which are markers of neuroendocrine tumors of intestinal origin, and glucagon (80%) and pancreatic polypeptide (100%) which are more typical of pancreatic NETs, was somewhat unexpected and underlines the heterogeneity of the disease and its differences from carcinoids of other districts.