Lck is a lymphoid-specific cytosolic protein tyrosine kinase (PTK), which is essential for T-cell development and function. It is constitutively associated with the cytoplasmic portions of the CD4 and CD8 surface receptors and plays a key role in T-cell antigen receptor (TCR)-linked signal transduction pathways. Association of the TCR with a peptide antigen-bound MHC complex facilitates the interaction of CD4 and CD8 with MHC class II and class I molecules, respectively, and thereby recruits the associated Lck to the vicinity of the TCR/CD3 complex. Lck then phosphorylates tyrosine residues within the immunoreceptor tyrosine-based activation motifs (ITAMs) in the cytoplasmic tails of the TCR-chains and CD3 subunits. The phospho-ITAMs serve as docking sites for Src homology domain 2 (SH2)-containing molecules, predominantly ZAP-70 and Syk. Only then can ZAP-70 undergo tyrosine phosphorylation, become enzymatically active and further phosphorylate downstream effector molecules. In addition, Lck contributes to signaling by other receptor molecules. Protein tyrosine kinase LCK is a non-receptor kinase belonging to the Src subfamily. LCK functions in a variety of ways, and therefore results in an assortment of diseases when not working properly. For example, T cell receptor binding activates LCK to induce downstream effects. In addition, LCK regulates chloride channel opening in lymphocytes. LCK is associated with various cancers and diseases, including neuroblastoma, Non-Hodgkins lymphoma, leukemia, SCID (severe combine immunodeficiency), and type I diabetes. Alternative splicing and differential promoter usage have been observed.
							
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