Among the four human gastroenteropancreatic neuroendocrine tumors (gastrinomas, insulinomas, tumors with carcinoid syndrome, functionally inactive neuroendocrine tumors), expression levels of SSTR1, SSTR5, and TGFBR1 and TGFBR2 (encoding TGF-β type 1 receptor, also termed activin receptor-like kinase 5, ALK5, and TβRII, respectively) varied significantly, suggesting the existence of different pathways during tumor subtype development (26). This evidence concerns the gene TGFBR1 and carcinoid syndrome.