Oncogenes in development.

Realization that the transforming oncogenes (v-onc) of the acutely oncogenic retroviruses are homologous to cellular genes (and were probably derived from them) brought several areas of research together with exciting prospects for advances in virology, carcinogenesis, evolution and development. The proto-oncogenes (c-onc) are likely to be crucially involved in growth regulation and/or differentiation because of their conservation throughout evolution and because of the well-known growth deregulation effects produced by the \-oncs. It was therefore reasoned that the normal counterpart of these genes should be active during embryonic development and that identification of a specific tissue or stage where c-oncs are expressed should help to identify their roles in all cells and provide a source of material to study the mechanisms of action. The preliminary results suggest growth modulatory roles for most oncogenes, and developmental studies have provided clues to c-onc roles that would not have been forthcoming from studies on cell lines. Several c-onc products have now been identified with specific cellular proteins, and these have confirmed their importance to growth regulation. They are growth factor or hormone receptors (such as c-erb-B, or epidermal growth factor [EGF] receptor; c-fms, or colony stimulating factor-1 [CSF-1] receptor and c-erb-A, or thyroid hormone receptor) or growth factors (such as c-sis, or B chain of the plateletderived growth factor [PDGF]). Table 1 lists the proto-oncogenes that have been studied in developing or differentiating systems. For general reviews, see Miiller, 1983; Hunter, 1984; Weinberg, 1984; Varmus, 1984; Heldin & Westermark, 1984; Sinkovics, 1984; Klein & Klein, 1985; Bishop, 1985; and Muller, 1986.