PDGF R alpha (platelet-derived growth factor receptor alpha) is a type I transmembrane glycoprotein in the class III subfamily of receptor tyrosine kinases (RTK) (1 ‑ 4). PDGF R alpha and PDGF R beta can form homo- or hetero-dimeric receptors when engaged by dimers of the PDGF family of growth factors, which include disulfide-linked homodimers of PDGF-A, B, C or D, or the heterodimer PDGF-AB that is mainly found in human platelets. While multiple in vitro ligand-receptor combinations have been identified, in vivo evidence indicates that PDGF R alpha primarily binds PDGF-AA and PDGF-CC, while PDGF R beta primarily binds PDGF-BB and probably PDGF-DD. Like all class III RTKs, the extracellular domain (ECD) of human PDGF R alpha (aa 24 ‑ 524) contains five immunoglobulin-like domains, while the intracellular region contains a split tyrosine kinase domain (aa 593 ‑ 954) (1 ‑ 4). Within the ECD, human PDGF R alpha shares 85%, 83%, 95%, 93%, and 88% aa sequence identity with mouse, rat, equine, canine and bovine PDGF R alpha respectively. PDGF R alpha autophosphorylates upon dimerization, activating signaling cascades in PI 3-kinase Ras‑MAP kinase, and PLC-gamma pathways (1, 2). Signaling is down‑regulated by SHP-2 phosphatase activity and by receptor endocytosis and lysosomal degradation. PDGF R alpha is expressed at low levels in most mesenchymal cells, but is strongly expressed in oligodendrocyte, lung, skin and intestinal progenitor cells and induced by inflammation or growth in culture (1 ‑ 4). During development, mesenchymal cells expressing PDGF R alpha respond to local gradients of epithelially produced PDGF-AA or PDGF-CC during formation of the cranial and cardiac neural crest, retina, gonads, lung alveoli, intestinal villi, skin, hair follicles, skeleton, teeth, palate, and interstitial kidney mesenchyme (1, 5). Deletion of PDGF R alpha in mice severely impairs mesenchymal derivatives in both embryo and extraembryonic tissues, and high or low PDGF R alpha signaling in humans may result in spina bifida or cleft palate‑type malformations. Postnatally, PDGF R alpha is implicated in gliomas and fibrotic disorders of lung, heart and skin (scleroderma) (6 ‑ 8).