GCG and neoplasm: As stated in the Introduction, the identification of specific tissue tumour markers still represents a huge medical need, since neither non-specific NET biomarkers (e.g., chromogranin-A (CHGA), Neuron Specific Enolase (NSE), Pancreatic Polipeptide (PP), Human Chorionic Gonadotropin (HCG), and Alpha Fetoprotein (AFP) [35]), or specific circulating NET biomarkers (e.g., gastrin, insulin, glucagone, somatostatin and vasoactive intestinal peptide (VIP)) have shown a validated clinical relevance [35].