Platelet-derived growth factor receptors (PDGFRs) and ligand patterns. Platelet-derived growth factor receptors (PDGFRs) and ligand patterns. PDGFRs are transmembrane proteins. The extracellular domain consists of five immunoglobulin-like domains; binding occurs at domains 2 and 3. The intracellular domain is a tyrosine kinase. There are three dimeric forms of PDGFRs (-αα, -ββ and -αβ). Five different ligand isoforms are known to bind to PDGFRs (AA, AB, BB, CC and DD). Rana Kanaan, and Charlie Strange Eur Respir Rev 2017;26:170061 ©2017 by European Respiratory Society