CSK(Tyrosine-protein kinase CSK) is also named as C-Src kinase,protein-tyrosine kinase CYL and belongs to the protein kinase superfamily. Csk is a non-receptor tyrosine kinase with a molecular mass of 50 kDa. It contains the SH3 and SH2 domains in its N-terminus and a kinase domain in its C-terminus. It negatively regulates the kinase activity of SFKs and plays an important role in physiological functions via signaling pathways for cell proliferation,differentiation,adhesion,and migration. Csk may play a role in tumor suppression by inhibiting the oncogenic activity of SFKs. Indeed,overexpression of Csk can inhibit tumor growth of human colon cancer cells. This protein can be autophosphorylated.