[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.

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.

PTPs are capable of modifying the activity of receptor tyrosine kinases in both a positive and negative manner.

[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.