 They play pivotal roles in diverse cellular activities including growth (by signaling neurotrophins), differentiation, metabolism, adhesion, motility, death.RTKs are composed of an extracellular domain, which is able to bind a specific ligand, a transmembrane domain, and an intracellular catalytic domain, which is able to bind and phosphorylate selected substrates.Binding of a ligand to the extracellular region causes a series of structural rearrangements in the RTK that lead to its enzymatic activation.