Receptor tyrosine kinases or RTKs are cell surface receptors that bind ligands, typically growth factors, and catalyze phosphate transfer from ATP to tyrosine residues in protein substrates. This way, RTKs transmit molecular signals through a cell to regulate growth, differentiation, and migration. Until bound by a ligand, RTKs remain monomeric and inactive. Ligand binding activates RTKs by different mechanisms. In ligand-induced dimerization, a dimeric ligand binds with two RTKs simultaneously to bring them closer. In receptor-mediated dimerization, a monomeric ligand binds each RTK and induces conformational changes to crosslink the two receptors together. Once dimerized, one RTK phosphorylates tyrosine residues on the other in a process called trans-autophosphorylation. The phosphorylated tyrosine residues can now bind target proteins that contain the Src homology 2 or SH2 domain, or the phospho-tyrosine binding or PTB domain. The RTKs phosphorylate these domains to relay the signal further downstream.