PDGF is a major serum mitogen that can exist as a homo- or heterodimeric protein consisting of disulfide-linked PDGF-A and PDGF-B chains. The PDGF-AA, PDGF-BB and PDGF-AB isoforms have been shown to bind to two distinct cell surface PDGF receptors with different affinities. Whereas PDGF Rα binds all three PDGF isoforms with high affinity, PDGF Rb binds PDGF-BB and AB, but not PDGF-AA. Both PDGF Ra and PDGF Rb are members of the class III subfamily of receptor tyrosine kinases (RTK) that also includes the receptors for M-CSF, SCF and Flt3 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. PDGF binding induces receptor homo-and heterodimerization and signal transduction. The expression of the alpha and beta receptors is independently regulated in various cell types. Only PDGF Ra is expressed in oligodendrocyte progenitor cells, mesothelial cell and liver endothelial cells. Soluble PDGF Ra has been detected in cell conditioned medium and human plasma. Recombinant soluble PDGF Ra binds PDGF with high affinity and is a potent PDGF antagonist (Heldin, C. H. and L. Claesson-Welsh, 1994, in Guidebook to Cytokines and Their Receptors, N. A. Nicola, editor, Oxford University Press, New York, New York, pp. 202-208).