The PDGF Receptor Type A (Alpha platelet-derived growth factor receptor precursor, CD140a antigen), a 170kD protein, binds all three isoforms of PDGF with high affinity whereas the PDGF Receptor Type B, a 190kD protein, appears to bind only the PDGF BB homodimer with high affinity. Both receptors are transmembranous, ligand activated protein tyrosine kinases, which phosphorylate a number of important signal transduction proteins, which are bound with differential affinities via SH2 domains. The response of any given cell to PDGF will depend on the types of receptors displayed on the surface and isoforms of PDGF present in the extracellular environment.