PDGF is a mitogen for mesenchyme- and glia-derived cells. It consists of two disulfide-bonded polypeptide chains, A and B, and occurs as three isoforms, PDGF AA, PDGF AB and PDGF BB. The three isoforms bind with different affinities to two receptor types, (alpha) and (beta), which are structurally related and endowed with protein-tyrosine kinase domains. Ligand binding induces activation of the receptor kinases by formation of receptor dimers; the A subunit of PDGF binds only to (alpha) receptors with high affinity, whereas the B subunit can bind to both (alpha) and (beta) receptors. Evidence suggests that PDGF may function as a neurotrophic factor.