Platelet-derived growth factor (PDGF) is a 30 kilodalton dimeric peptide comprising A and B chains. The two types of PDGF receptors (alpha and beta) that have been identified belong to the protein-tyrosine kinase family of growth factor receptors. PDGF is a potent mitogen for a variety of cell types, and in gene knock-out experiments has been shown to be required for the development of inter alia mesangial cells of renal glomeruli and smooth muscle cells of lung alveolar septa. PDGF is believed to fulfil important autocrine and paracrine growth-related functions, and its overproduction has been recognised in various malignant tumours and in non-neoplastic hyperproliferative disorders.