[1] Receptor tyrosine kinases have been shown not only to be key regulators of normal cellular processes but also to have a critical role in the development and progression of many types of cancer.

[2] Mutations in receptor tyrosine kinases lead to activation of a series of signalling cascades which have numerous effects on protein expression.

[8] The extracellular N terminal region exhibits a variety of conserved elements including immunoglobulin (Ig)-like or epidermal growth factor (EGF)-like domains, fibronectin type III repeats, or cysteine-rich regions that are characteristic for each subfamily of RTKs; these domains contain primarily a ligand-binding site, which binds extracellular ligands, e.g., a particular growth factor or hormone.

Through diverse means, extracellular ligand binding will typically cause or stabilize receptor dimerization.

Phosphorylation and activation of these two proteins on receptor binding lead to the initiation of signal transduction pathways.

[2] An example of a vital signal transduction pathway involves the tyrosine kinase receptor, c-met, which is required for the survival and proliferation of migrating myoblasts during myogenesis.

This local action of FGFs (Fibroblast Growth Factors) with their RTK receptors is classified as paracrine signalling.

[12] In mice, loss of signaling by any member of the ErbB family results in embryonic lethality with defects in organs including the lungs, skin, heart, and brain.

[18] The VEGF receptors have an extracellular portion consisting of seven Ig-like domains so, like FGFRs, belong to the immunoglobulin superfamily.

RET is the receptor for members of the glial cell line-derived neurotrophic factor (GDNF) family of extracellular signalling molecules or ligands (GFLs).

[23] The receptor tyrosine kinase (RTK) pathway is carefully regulated by a variety of positive and negative feedback loops.

[25] Protein Tyrosine Phosphatase (PTPs) are a group of enzymes that possess a catalytic domain with phosphotyrosine-specific phosphohydrolase activity.

[26] PTPs can dephosphorylate the activated phosphorylated tyrosine residues on the RTKs[27] which virtually leads to termination of the signal.

Cd45, a cell surface glycoprotein, plays a critical role in antigen-stimulated dephosphorylation of specific phosphotyrosines that inhibit the Src pathway.

[30] Herstatin is an autoinhibitor of the ErbB family,[31] which binds to RTKs and blocks receptor dimerization and tyrosine phosphorylation.

[33] Herceptin, a monoclonal antibody that is capable of binding to the extracellular domain of RTKs, has been used to treat HER2 overexpression in breast cancer.