Spleen Tyrosine Kinase （SYK） belongs to the family of non-receptor type Tyr protein kinases. The SYK protein is extensively expressed in hematopoietic cells and is involved in coupling activated immunoreceptors to downstream signaling events which mediate diverse cellular responses， including proliferation， differentiation， and phagocytosis. SYK is considered to be a modulator of epithelial cell growth and a likely tumor suppressor in human breast carcinomas. SYK is a positive effector of BCR-stimulated responses. SYK connects the B-cell antigen receptor （BCR） to the utilization of calcium ion 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-348 and Tyr-352. Therefore the differential phosphorylation of Syk can define the pathway by which BCR is coupled to the regulation of intracellular calcium ion （By similarity）. SYK also phosphorylates USP25 and regulates its intracellular levels.
