Biomarkers with potential prognostic significance include epidermal growth factor receptor (EGFR), somatostatin receptor (SSR) or programmed death ligand-1 (PD-L1) in pulmonary subtype and glucose transporters type 1 (GLUT-1), O-6 -methylguanine-DNA methyltransferase (MGMT) for pancreatic NETs, as well as connective tissue growth factor for carcinoid heart disease (CCN2) or vascular endothelial growth factor receptor (VEGFR) for gastrointestinal NET [39] (Figure 1). Here, MGMT is linked to carcinoid heart disease.