PDGF-BB (Platelet-Derived Growth Factor BB) is a homodimeric molecule expressed by hepatocytes and nonresorbing osteoclasts, and it promotes osteoblast development and bone formation. It is also produced by platelets, macrophages, and mast cells. At sites of injury, it promotes neutrophil and macrophage infiltration for debridement, fibroblast secretion of new extracellular matrix, and IGF-I-mediated re-epithelialization. The traditional receptor for PDGF is either a homodimer or heterodimer created from two type I transmembrane RTKs, PDGF R alpha and PDGF R beta. PDGF-BB has been shown to bind the alpha/alpha homodimer, alpha/beta heterodimer, and the beta/beta homodimer in vitro, and act through the beta/beta homodimer in vivo.