PDGFs are disulfide-linked dimers consisting of two 12.0-13.5kD polypeptide chains, designated PDGF-A and PDGF-B chains. The three naturally occurring PDGFs; PDGF-AA, PDGF-BB and PDGF-AB, are potent mitogens for a variety of cell types including smooth muscle cells, connective tissue cells, bone and cartilage cells, and some blood cells. The PDGFs are stored in platelet a-granules and are released upon platelet activation. The PDGFs are involved in a number of biological processes, including hyperplasia, chemotaxis, embryonic neuron development, and respiratory tubule epithelial cell development. Two distinct signaling receptors used by PDGFs have been identified and named PDGFR-a and PDGFR-b. PDGFR-a is high-affinity receptor for each of the three PDGF forms. On the other hand, PDGFR-b interacts with only PDGF-BB and PDGF-AB. Recombinant human PDGF-AA is a 28.5kD disulfide-linked homodimer of two A chains (250 total amino acids).