الفهرس 
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Abstract
Cells are excited when extra cellular signaling molecules bind to a receptor each receptor identifies a specific protein (ligand) Receptors act as transducers that transform the signal from one physical form to another. Most signaling molecules cannot throw through the cell membrane (Their receptors are in the cell membrane). Small hydrophobic signal molecules can dispersed directly into the cell cytoplasm (Their receptors are cytoplasmic or nuclear) [1]. Signaling molecules are tow type: Hormones (Insulin, Cortisol,etc.) and Growth factors (Epidermal Growth Factor(EBGF),Platelet Derived Growth Factor(PDGF),Fibroblast Growth Factor (FGF),Tumor necrosis factor (TNF) and Cytokines [2]. There are multiple ―superfamilies‖ of growth factors that include multiple subfamilies of proteins, completely with related primary sequences. There are large lists of various well-known growth factor superfamilies. Such superfamilies may themselves include several subfamilies, each with multiple submembers (each of which is encoded by a distinct gene) [3]. For instance, the fibroblast growth factor (FGF) superfamily contains at least 18 distinct members. The transforming growth factor (TGFβ) superfamily contains at least 35 known members that fall into about 10 subfamilies. One of these subfamilies, the bone morphogenic proteins are comprised of at least 15 different gene products.The neurotrophin superfamily contains 4 members. Also, there are others lists a group of additional growth factors that have been found to play important roles in development [3]. Growth factors are ligands for transmembrane receptors. Each growth factor superfamily has a corresponding family of related receptors. There is high specificity with respect to receptor binding between superfamilies, but there are cases in which more than one family member binds to a single receptor and in which a given family member binds to multiple receptors. For instance, there are 4 FGF receptors for the 18 members of the FGF superfamily[4]. Figure 1 shows the binding of the neurotrophin family to their receptors Tyrosine kinase receptor (Trks) and shows that a single ligand can bind only a single receptor, that a certain ligand can bind more than one receptor and that one receptor can bind several diverse ligands.