Non-receptor tyrosine-protein kinase that plays an important role in the regulation of cell growth, differentiation, migration and immune response. Phosphorylates tyrosine residues located in the C-terminal tails of Src-family kinases (SFKs). Upon tail phosphorylation, Src-family members engage in intramolecular interactions between the phosphotyrosine tail and the SH2 domain that result in an inactive conformation. To inhibit SFKs, CSK is recruited to the plasma membrane via binding to transmembrane proteins or adapter proteins located near the plasma membrane (By similarity). ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Homodimer (via SH3-domain). The architecture of this protein is similar to that of Src-family kinases (SFKs) with one N-terminal SH3 domain, one SH2 domain, and a C-terminal kinase domain. Phosphorylated at Ser-364 by PKA, leading to increased activity. Autophosphorylated (By similarity). Belongs to the protein kinase superfamily. Tyr protein kinase family. CSK subfamily.