Somatostatin is a 14 or 28 amino acid regulatory peptide that inhibits hormone secretion from the pituitary, pancreas, and other endocrine sites. A family of 6 GPCRs, SST1, SST2A, SST2B, SST3, SST4 and SST5 , mediate the biological activity of somatostatins. The somatostatin receptors couple to Gi to inhibit cAMP production, and also increase MAP kinase signalling. Several tumors have been shown to overexpress somatostatin receptors, and binding of somatostatin to these tumor cells stimulates or inhibits proliferation, depending on the receptor subtypes expressed. However, SST3 appears to promote apoptosis, and expression of SST3 was found to be lower in gastric cancer cells than in normal gastric mucosa, in proportion to susceptibility to apoptosis induced by somatostatin analogs. In addition, SST3 is expressed in smooth muscle cells and mediates gastrointestinal contractions.