Catalytic domain of the Protein Tyrosine Kinase, Spleen tyrosine kinasePTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. Syk is a cytoplasmic (or nonreceptor) PTK containing two Src homology 2 (SH2) domains N-terminal to the catalytic tyr kinase domain. Syk was first cloned from the spleen, and its function in hematopoietic cells is well-established. It is involved in the signaling downstream of activated receptors (including B-cell and Fc receptors) that contain ITAMs (immunoreceptor tyr activation motifs), leading to processes such as cell proliferation, differentiation, survival, adhesion, migration, and phagocytosis. More recently, Syk expression has been detected in other cell types (including epithelial cells, vascular endothelial cells, neurons, hepatocytes, and melanocytes), suggesting a variety of biological functions in non-immune cells. Syk plays a critical role in maintaining vascular integrity and in wound healing during embryogenesis. It also regulates Vav3, which is important in osteoclast function including bone development. In breast epithelial cells, where Syk acts as a negative regulator for EGFR signaling, loss of Syk expression is associated with abnormal proliferation during cancer development suggesting a potential role as a tumor suppressor. In mice, Syk has been shown to inhibit malignant transformation of mammary epithelial cells induced with murine mammary tumor virus (MMTV). The Syk subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.