PDGF isoforms exert their cellular effects by binding to the structurally similar receptor tyrosine kinases (RTK), PDGF R alpha (CD140a) and PDGF R beta (CD140b). PDGF-AA, PDGF-BB, PDGF-CC, PDGF-DD and PDGF-AB isofoms bind the two distinct cell surface PDGF receptors with different affinities. Both PDGF R alpha and PDGF R beta are members of the class III subfamily of RTK which 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 leading to signal transduction. The expression of PDGF alpha and beta receptors is independently regulated in various cell types.