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Platelet-derived growth factor （PDGF） is a family of three disulfide-linked glycoprotein dimers of M.W. 29kDa-32kDa that arise from astochastic assembly of the homologous subunits, A-chain and B-chain, yielding the heterodimer PDGF-AB and homodimers PDGF-AA and PDGF-BB. PDGF-AA and its receptor represent an important epithelial-mesenchymal interaction which plays a critical role in early lung branching morphogenesis.