Cell proliferation and directed cell movement are key events in several normal processes including embryogenesis and development, responses to injury such as wound repair, and in those tissues that maintain themselves by continual or intermittent cell turnover, such as the epithelia that line all body surfaces and cavities, and the cells of the hematopoietic system. Abnormal cell proliferation and its control are primary problems in a number of diseases, including neoplasia and atherosclerosis. In many instances, the basis for this cell proliferation, be it normal or abnormal, probably lies in the response of the cells to a group of polypeptide hormones, or growth factors, which may act singly or in concert to stimulate the proliferation of given populations of cells. Each growth factor appears to have specific target cells and tissues. The discovery of one of the most ubiquitous of these factors, plateletderived growth factor (PDGF), came in 1974 when it was observed that material released from platelets was the principal source of mitogens present in whole blood serum, and was responsible for the growth of many cells in culture that are serum-dependent (Ross et al., 1974; Kohler and Lipton, 1974).

PDGF from human platelets is a cationic glycoprotein of approximately 30,000 M,(Antoniades, 1981; Deuel et al., 1981; Heldin et al., 1981a; Raines and Ross, 1982). Reduction of disulfide bonds destroys its mitogenic activity, and generates multiple protein species of 14,000 to 17,000 Mr. Sequence analysis revealed two distinct but related sequences in these multiple species (Antoniades and Hunkapiller, 1983; Waterfield et al., 1983) suggesting that PDGF from human platelets is a heterodimer of two chains; these chains have been termed A and B (Johnsson et al., 1984). In contrast, porcine PDGF appears to consist of BB homodimers (Stroobant and Waterfield, 1984) while PDGF produced by osteosarcoma cells consists of AA homodimers (Heldin et al., 1986). Hence, both A chain and B chain homodimers can be mitogenic. Previous reviews of PDGF have summarized our knowledge of the molecule itself (Ross and Vogel, 1978; Scher et al., 1979; Deuel and Huang, 1983; Westermark et al., 1983) and its relation to the v-sis oncogene of simian sarcoma virus (SSV)(Stiles, 1983; Deuel and Huang, 1984; Heldin and Westermark, 1984). This review will discuss what we have recently learned about the biogenesis of PDGF, and its possible roles in normal biology and in disease.