Catalytic domain of the Protein Tyrosine Kinase, C-terminal Src kinasePTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Csk catalyzes the tyr phosphorylation of the regulatory C-terminal tail of Src kinases, resulting in their inactivation. Csk is expressed in a wide variety of tissues. As a negative regulator of Src, Csk plays a role in cell proliferation, survival, and differentiation, and consequently, in cancer development and progression. Csk is a cytoplasmic (or nonreceptor) PTK containing the Src homology domains, SH3 and SH2, N-terminal to the catalytic tyr kinase domain. To inhibit Src kinases, Csk is translocated to the membrane via binding to specific transmembrane proteins, G-proteins, or adaptor proteins near the membrane. In addition, Csk also shows Src-independent functions. It is a critical component in G-protein signaling, and plays a role in cytoskeletal reorganization and cell migration. The Csk subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.