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The platelet-derived growth factor (PDGF) family consists of proteins derived from four genes (PDGF-A, -B, -C, and -D) that form disulfide-linked homodimers (PDGF-AA, -BB, -CC, and -DD) and a heterodimer (PDGF-AB) (1, 2). These proteins regulate diverse cellular functions by binding to and inducing the homo- or hetero-dimerization of two receptors (PDGF R alpha  and R beta ). Whereas alpha / alpha homo-dimerization is induced by PDGF-AA, -BB, -CC, and -AB, alpha / beta  hetero-dimerization is induced by PDGF-AB, -BB, -CC, and -DD, and beta / beta  homo-dimerization is induced only by PDGF-BB and -DD (1-4). Both PDGF R alpha  and R beta are members of the class III subfamily of receptor tyrosine kinases (RTK) that also includes the receptors for M-CSF, SCF, and Flt-3 ligand. All class III RTKs are characterized by the presence of five immunoglobulin-like domains in their extracellular region and a split kinase domain in their intracellular region. Ligand-induced receptor dimerization results in autophosphorylation in trans resulting in the activation of several intracellular signaling pathways that can lead to cell proliferation, cell survival, cytoskeletal rearrangement, and cell migration. Many cell types, including fibroblasts and smooth muscle cells, express both the alpha  and beta  receptors. Others have only the alpha  receptors (oligodendrocyte progenitor cells, mesothelial cells, liver sinusoidal endothelial cells, astrocytes, platelets, and megakaryocytes) or only the  beta receptors (myoblasts, capillary endothelial cells, pericytes, T cells, myeloid hematopoietic cells, and macrophages) (1, 2). Recombinant mouse and human soluble PDGF R beta  bind PDGF with high affinity and are potent PDGF antagonists.