Lck is a 56 kD tyrosine protein kinase and a member of the src subfamily that contains both an SH2 and SH3 domain. Alternatively spliced isoforms of Lck have been reported. Lck is a cytoplasmic protein bound to CD4 and CD8 in T lymphocytes that participates in antigen-induced T cell activation through phosphorylation of the T cell receptor zeta chain. Lck is activated upon T cell receptor engagement and is involved in the onset of cell cycle progression induced by interleukin-2. Phosphorylation by Csk downregulates the activity of Lck. In addition to CD4 and CD8, Lck has been reported to interact with PI3K through its SH3 domain and tyrosine phosphorylated KHDRBS1/p70 through its SH2 domain. In addition, Lck has been shown to associate with the SAP transmembrane tyrosine phosphatase. The Lck-01 monoclonal antibody recognizes human Lck and has been shown to be useful for Western blotting, immunoprecipitation, immunocytochemistry, and flow cytometry.