IGF-I, IGF-II, insulin-like growth factors -1 and -2 are important regulators of signaling that mediates growth and metabolism in cells. Insulin-like growth factor -1 shares structural homology with proinsulin and is mainly produced by liver. IGF-1 acts via specific receptor, IGF-1R, a heterodimeric tyrosine kinase receptor that signals to various second messenger pathways such as MAPK, Raf/Ras and PI3K cascades.The receptors are ligand binding factors of type 1, 2 or 3 and protein-molecules that receive chemical-signals from outside a cell. When such chemical-signals couple or bind to a receptor, they cause some form of cellular/tissue-response, e.g. a change in the electrical-activity of a cell. In this sense, am olfactory receptor is a protein-molecule that recognizes and responds to endogenous-chemical signals, chemokinesor cytokines e.g. an acetylcholine-receptor recognizes and responds to its endogenous-ligand, acetylcholine. However, sometimes in pharmacology, the term is also used to include other proteins that are drug-targets, such as enzymes, transporters and ion-channels.