The Src family of protein tyrosine kinases, which includes Src, Lyn, Fyn, Yes, Lck, Blk, and Hck, are important in the regulation of growth and differentiation of eukaryotic cells. Src activity is regulated by tyrosine phosphorylation at two sites, but with opposing effects. While phosphorylation at Tyr416 in the activation loop of the kinase domain upregulates enzyme activity, phosphorylation at Tyr527 in the carboxy-terminal tail by Csk renders the enzyme less active. The kinase Fyn plays a role in T-cell receptor signaling and also helps in memory formation and adhesion mediated signaling. Lymphocyte specific kinase Lck is essential in the differentiation and activation of T-cells. Hematopoietic Lyn kinase, also phosphorylated by Csk, is involved in the regulation of B-cell function, migration and development. The ubiquitously expressed kinase Yes acts downstream of several different cell surface receptors, including G-protein-coupled receptors, and is involved in the regulation of angiogenesis, the cell cycle and cell adhesion.