Abstract
The possibility that certain tumor viruses are simply laboratory artifacts makes them in some respects more interesting rather than less, because it means that they may be offering us a way of isolating and studying the genes responsible for the cancerous state. That seems to be the real justification for putting so much effort into investigating the tumor viruses, not the vague hope that human cancer will turn out to be a virus disease.1 RNA tumor viruses (or "retroviruses") are proved agents of oncogenesis in certain species.2 It is therefore not surprising that in recent years there has been a . . .