G-protein coupled receptor 40 (GPR40 or FFA1) is specifically expressed in brain and pancreas. In pancreas, abundant GPR40 is localized to insulin-producing beta cells. Long-chain FFAs amplify glucose-stimulated insulin secretion from pancreatic beta cells by activating GPR40, indicating that GPR40 agonists and/or antagonists have potential for the development of new anti-diabetic drugs. GPR40 overexpression in breast cancer cells amplified oleate-induced proliferation, whereas silencing the GPR40 gene decreased it. These results suggest that GPR40 is implicated in the control of breast cancer cell growth by fatty acids and that GPR40 may provide a link between fat and cancer.