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Anaplastic Lymphoma Kinase (ALK) is a [https://en.wikipedia.org/wiki/Transmembrane_protein transmembrane] receptor and a member of the family of [https://proteopedia.org/wiki/index.php/Receptor_tyrosine_kinases Receptor Tyrosine Kinases (RTKs)]<ref name="Iwahara">PMID:9053841</ref>. RTKs are a family of biomolecules that are primarily responsible for biosignaling pathways such as the insulin signaling pathway. ALK was identified as a novel tyrosine phosphoprotein in 1994 in an analysis of [https://lymphoma.org/aboutlymphoma/nhl/alcl/ Anaplastic Large-Cell Lymphoma], the protein's namesake.<ref name ="Huang" /> A full analysis and characterization of ALK was completed in 1997, properly identifying it as a RTK, and linking it closely to [https://en.wikipedia.org/wiki/Leukocyte_receptor_tyrosine_kinase Leukocyte Tyrosine Kinase] (LTK).<ref name ="Huang" /> ALK's normal activity as a receptor tyrosine kinase is to transfer a gamma-phosphate group from adenosine triphosphate (ATP) to a tyrosine residue on it's substrate.<ref name ="Huang" /> ALK is one of more than 50 RTKs encoded within the human genome, <ref name ="Huang" /> and it's tyrosine kinase activity seems to be especially important in the developing nervous system. <ref name ="Huang" /> ALK is most commonly associated with oncogenesis, as various factors, including overstimulation, lead to extreme cell proliferation. It is primarily found in the fetal and infant developing nervous system, however when associated with cancer it can be found in systems other than the nervous system. Examples of these include colon and prostate cancer, where ALK is not normally expressed. This particular protein is interesting in part due to its unique structure, of that it shares the most similarity with LTKs, and it's popular association with devastating cancers.

ALK is a close homolog of LTK, and together these two homologues constitute a subgroup within the superfamily of [https://proteopedia.org/wiki/index.php/Insulin_receptor insulin receptors]<ref name="Della Corte" />. ALK is composed of three primary regions: the extracellular region, the transmembrane region, and the intracellular region. [[Image:Full ALK Structure Graphic.PNG|600 px|right|thumb|Figure 1. Overview of Anaplastic Lymphoma Kinase Structure with domains where known structure are color coordinated and other domains are grayed out. The abbreviations are as follows: NTR-N-terminal Region, MAM-Meprin-A-5 protein-receptor protein tyrosine phosphatase μ, LDL-low-density lipoprotein receptor class A, THB-three helix bundle-like, GlyR-poly-glycine region, TNF-tumor necrosis factor-like domain, EGF-epidermal growth factor-like, TMH-transmembrane helix]] The extracellular region of ALK contains 8 total domains within 2 fragments. A Three Helix Bundle-like domain (THB-like), a Poly-Glycine domain (GlyR), a Tumor Necrosis Factor-like domain (TNF-like), and an Epidermal Growth Factor-like domain (EGF-like) make up the ligand binding fragment while a N-terminal domain, two [https://en.wikipedia.org/wiki/Meprin_A meprin–A-5] protein–receptor protein tyrosine phosphatase μ (MAM) domains and a [https://en.wikipedia.org/wiki/Low-density_lipoprotein low-density lipoprotein] receptor class A (LDL) domain sandwiched between the two MAM domains make up the second fragment. All four domains of the ligand binding fragment of the extracellular region contribute to ligand-binding <ref name ="Huang" />. The presence of an LDL domain sandwiched by two MAM domains is a unique feature that ALK does not share with other RTKs. The purpose behind this unique difference is still unclear, but the MAM region has been hypothesized to play a role in cell-cell signaling<ref name="Palmer">PMID:19459784</ref>. The [https://en.wikipedia.org/wiki/Transmembrane_domain transmembrane helical region] (TMH) bridges the gap between the intracellular and extracellular regions. The intracellular tyrosine kinase region features the Kinase domain and the C-terminal end (Figure 1).