Lyn (also known as p53/56 Lyn) is a membrane-associated protein tyrosine kinase (PTK) mostly expressed in hemopoietic cells which is important in cellular signaling. It contains an SH2 and SH3 domain and has been found to be cleaved after activation of caspases in apoptosis. A member of the Src family of PTKs, there are two known isoforms for Lyn which plays an indispensable role in the Fc epsilon RI (Fcer1) and the B-cell IgM receptor signaling pathway and is essential for Syk activation and Lat phosphorylation after Fcer1 aggregation and can also phosphor-ylate Tec on multiple residues. Lyn can also be regulated by IL-2 and IL-3.Lyn is a member of the src family of non-receptor protein tyrosine kinases that is predominantly expressed in haematopoietic tissues. Like all members of the src family, lyn is thought to participate in signal transduction from cell surface receptors that lack intrinsic tyrosine kinase activity. It is associated with a number of cell surface receptors including the B cell antigen receptor and immunoglobulin E receptor (FceRI).