Catalytic domain of the Protein Tyrosine Kinases, Platelet Derived Growth Factor ReceptorsPTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. The PDGFR subfamily consists of PDGFR alpha, PDGFR beta, KIT, CSF-1R, the mammalian FLT3, and similar proteins. They are receptor PTKs (RTKs) containing an extracellular ligand-binding region with five immunoglobulin-like domains, a transmembrane segment, and an intracellular catalytic domain. PDGFR kinase domains are autoinhibited by their juxtamembrane regions containing tyr residues. The binding to their ligands leads to receptor dimerization, trans phosphorylation and activation, and intracellular signaling. PDGFR subfamily receptors are important in the development of a variety of cells. PDGFRs are expressed in a many cells including fibroblasts, neurons, endometrial cells, mammary epithelial cells, and vascular smooth muscle cells. PDGFR signaling is critical in normal embryonic development, angiogenesis, and wound healing. Kit is important in the development of melanocytes, germ cells, mast cells, hematopoietic stem cells, the interstitial cells of Cajal, and the pacemaker cells of the GI tract. CSF-1R signaling is critical in the regulation of macrophages and osteoclasts. Mammalian FLT3 plays an important role in the survival, proliferation, and differentiation of stem cells. The PDGFR 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 .