In virtually all cases where the function of a particular pro tooncogene has been elucidated, the encoded product has been identified as a key regulatory element in the signal transduction pathways that control cell growth and differentiation. Retroviral and cellular oncogenes are perverted variants of these genes; their products are qualitatively or quantitatively abnor mal and cause transformation by upsetting the growth-control ling circuits. The literature has in recent years been flooded by examples that support this unifying concept of cell growth and transformation. From a historical point of view, the first ex ample was the discovery that the normal counterpart of the oncogene\-sis of simian sarcoma virus is the cellular gene encoding the B-chain of PDGF2 (1-3). Byand large, the mech anism of action of the sis gene in cell transformation has subsequently been clarified. Thus, it has been unequivocally proven that transformation by the sis gene occurs by an auto crine mechanism (4-6). However, there are also questions that remain to be answered which concern important aspects of the function of PDGF and the sis gene in transformation and tumor development. We will in this paper highlight some of the structural and functional properties of PDGF, its receptors, and signal transduction pathways with special emphasis on cell transformation and oncogenesis.