Since the discovery of v-jun as the transforming protein of the sarcoma virus 17 three mammalian homologues of v-jun have been isolated. All three jun genes respond to a multitude of agents, and the encoded proteins in turn bind to and regulate, positively or negatively, the transcription of dependent genes, thereby influencing cellular fate. In addition, through transcription factor "cross-talk" Jun influences the transcription of genes regulated by different classes of transcription factors, such as steroid hormone receptors. Although the role of Jun proteins in transcriptional regulation has been thoroughly analyzed in recent years, the role of Jun proteins in oncogenesis is still poorly understood.