raf Family Serine/Threonine Protein Kinases in Mitogen Signal Transduction

raf family genes encode signal transducers that first were identified in their altered form as oncogenes (Rapp et al. 1983a,b, 1988). Oncogenes are constitutively activated versions of genes encoding links in signal transduction pathways that feed into mitogenesis. In addition, they may regulate events unrelated to mitogenesis in terminally differentiated tissues. To date, raf family protein kinases remain the only widely expressed class of serine/threonine-specific kinases from which full-fledged oncogenes have emerged. The only other known serine/threonine protein-kinase-derived oncogene, mos, is expressed in only a few tissues (Seth and van de Woude 1988).