The members of the Src-family kinases are Src, Lyn, Fyn, Yes, Hck, Lck, Fgr, Blk, and Yrk. Each of these have a common structure consisting of an unique domain at the N-terminal, followed by SH3, SH2 and tyrosine kinase domains. In immume cells, the Src-family kinases play roles as critical regulators of a large number of intracellular signaling pathways, including integrin signaling pathway. Integrins are major cellular receptor that mediate cell to cell and cell to substratum interactions. Lck is expressed almost exclusively in T cells and interacts with cytoplasmic regions of CD4 and CD8 coreceptor molecules, and thus plays an important role in relaying TCR-mediated activation signal. Lck is regulated by phosphorylation of its Tyr394 and Tyr505. Tyrosine kinase that plays an essential role for the selection and maturation of developing T-cell in the thymus and in mature T-cell function. 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 tyrosines residues within the immunoreceptor tyrosines-based activation motifs (ITAMs) in the cytoplasmic tails of the TCRγ chains and CD3 subunits, initiating the TCR/CD3 signaling pathway. In addition, contributes to signaling by other receptor molecules. Associates directly with the cytoplasmic tail of CD2, and upon engagement of the CD2 molecule, LCK undergoes hyperphosphorylation and activation. Also plays a role in the IL2 receptor-linked signaling pathway that controls T-cell proliferative response. Binding of IL2 to its receptor results in increased activity of LCK. Is expressed at all stages of thymocyte development and is required for the regulation of maturation events that are governed by both pre-TCR and mature α β TCR.