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 TGFBR2 and digestive system neuroendocrine tumor, grade 1/2.