Schematic diagram showing PDGFR-PDGF interactions in vitro: PDGF ligand dimers bind either PDGFRα or PDGFRβ homodimers or the α/β heterodimer. Arrows show proven in vitro ligand-receptor interactions. Each PDGF receptor has five extracellular immunoglobulin-like domains and two intracellular tyrosine kinase domains. PDGFR chains are shown in blue (PDGFRα) and red (PDGFRβ). PDGF ligands are shown in green (PDGF-A), yellow (PDGF-B), orange (PDGF-C) and purple (PDGF-D).