GPR40 (Free fatty acid receptor 1, FFA1) is a class A G-protein coupled receptor that in humans is encoded by the FFAR1 gene. It is strongly expressed in the cells of the pancreas and to a lesser extent in the brain. This membrane protein binds free fatty acids, acting as a nutrient sensor for regulating energy homeostasis. GPR40 is activated by medium to long chain fatty acids. GPR40 is most strongly activated by eicosatrienoic acid, but has been found to be activated by fatty acids as small as 10 carbons long. For saturated fatty acids the level of activation is dependent on the length of the carbon chain, which is not true for unsaturated fatty acids. It has been found that three hydrophilic residues (arginine-183, asparagine-244, and arginine-258) anchor the carboxylate group of a fatty acid, which activates GPR40.