Lyn (V-yes-1 Yamaguchi sarcoma viral related oncogene homolog) is a member of the Src family of protein tyrosine kinases and is expressed primarily in hematopoietic cells and neural tissues. Lyn is involved in the regulation of cell activation by undergoing phosphorylation upon activation. Lyn activation triggers downstream signaling pathways that are mediated by Lyn phosphorylation of tyrosine residues of receptor proteins and recruitment and activation of other kinases including Syk, phospholipase Cγ2 (PLCγ2) and phosphatidyl inositol-3 kinase. Lyn also plays an important role in B cells as a key regulator of both positive and negative signals, thus acting to maintain the balance between activation and inhibition. Deregulation of this balance can result in severe pathological consequences such as autoimmune disease.