Tyrosine kinase that plays an essential role for the selection and maturation of developing T-cell in the thymus and in mature T-cell function. Is constitutively associated with the cytoplasmic portions of the CD4 and CD8 surface receptors and plays a key role in T-cell antigen receptor(TCR)-linked signal transduction pathways (By similarity). ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Inhibited by tyrosine phosphorylation. Binds to the cytoplasmic domain of cell surface receptors, such as CD4, CD8. Phosphorylated on Tyr-393, which increases enzymatic activity, this site is dephosphorylated by PTN22. Phosphorylated on Tyr-504, presumably by CSK, which decreases activity. Dephosphorylated by PTPRC/CD45. Dephosphorylation at Tyr-393 by PTPN2 negatively regulates T-cells differentiation (By similarity). Palmitoylation regulates association with the plasma membrane. Belongs to the protein kinase superfamily. Tyr protein kinase family. SRC subfamily.