Tyrosine-protein kinase CSK also known as C-terminal Src kinase is an enzyme that, in humans, is encoded by the CSK gene. This enzyme phosphorylates tyrosine residues located in the C-terminal end of Src-family kinases (SFKs) including SRC, HCK, FYN, LCK, LYN and YES1. This Non-receptor tyrosine-protein kinase plays an important role in the regulation of cell growth, differentiation, migration and immune response. CSK acts by suppressing the activity of the Src family of protein kinases by phosphorylation of Src family members at a conserved C-terminal tail site in Src. Upon phosphorylation by other kinases, 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 then recruited to the plasma membrane via binding to transmembrane proteins or adapter proteins located near the plasma membrane and ultimately suppresses signaling through various surface receptors, including T-cell receptor (TCR) and B-cell receptor (BCR) by phosphorylating and maintaining inactive several effector molecules.