SYK: spleen tyrosine kinase. Syk (72 kDa) is a non-receptor protein tyrosine kinase that plays an important role in immune receptor signal transduction and is implicated in endothelial cell functions, including cell growth and migration. SYK is a positive effector of BCR stimulated responses. It couples the B cell antigen receptor (BCR) to the mobilization of calcium ions either through a phosphoinositide 3 kinase dependent pathway, when not phosphorylated on tyrosines of the linker region, or through a phospholipase C gamma dependent pathway, when phosphorylated on Tyr 342 and Tyr 346. Thus the differential phosphorylation of SYK can determine the pathway by which BCR is coupled to the regulation of intracellular calcium ions.