If tumor viruses did not exist in nature they might have been created by scientists interested in basic mechanisms of develop- ment, differentiation, and tumorigenesis. In contemporary euka- ryotic cell biology tumor viruses playa similar role to that which bacteriophages once had for the molecular biology of prokary- otes. Tumor viruses provide extremely useful probes for the above cellular processes since their life cycle is genetically pro- grammed and can be followed at DNA, RNA, and protein levels. The experimental systems reviewed in this volume utilize a wide variety of viruses. A...