PKOBLEMS CONCERNING THE TUMOR VIRUSES 343 



within the past few years, the virus suddenly disappeared from two members 

 of the Kill high-cancer strain (Andervont, 1958). The disappearance was 

 observed within two generations of its occurrence because of the high 

 incidence of tumors in the strain, and close descendants of the mice from 

 which the virus disappeared were available for study. This rinding has 

 confirmed the observation of Murray and Warner. 



The chief reason for mentioning these deviations in the activity of the virus 

 is to draw attention to the fact that they occur rarely in inbred mice. This 

 could almost be expected because, similar to the Rous virus, the virus of 

 mouse mammary cancer has not only been maintained in the laboratory for 

 many years, but its hosts have been inbred intensively for susceptibility to 

 its activity. Perhaps the establishment of inbred strains by selection toward 

 susceptibility to the virus may have resulted in the unique host-virus 

 relationship which permits detection of the virus long before the host has 

 cancer. 



In order to ascertain whether this relationship was restricted to inbred 

 hosts and, also, whether the virus was involved in the natural disease, it was 

 thought advisable to search for the virus in wild house mice. The necessity 

 for the latter study stemmed from the knowledge that tumors arose in inbred 

 mice and in their hybrids in the absence of the virus. Two reports (Andervont, 

 1952; Andervont and Dunn, 1956) contain details of the methods used and 

 the results obtained in this investigation. The virus was exposed in wild 

 house mice by having them suckle virus-free inbred mice which were very 

 susceptible to the virus. The occurrence of tumors in the inbred mice indicated 

 that the wild mouse virus was low in activity or in amount because the 

 fostered animals developed few tumors. Wild mice suckled by inbred mice 

 carrying a potent virus did develop tumors but showed a lower incidence 

 than inbred mice exposed to the same virus. These findings permitted the 

 conclusion that the wild house mice carried a virus of low activity which was 

 transmitted through successive generations of relatively resistant wild mice. 

 Recent studies (Andervont, 1958) have shown that the wild mouse virus 

 increased in activity after passage through twenty generations of inbred 

 mice. Thus, three strains of the virus are available: one, low in activity or 

 amount and carried by wild mice; another, of medium activity after twenty 

 generations of passage in inbred mice; another, the more familiar virus, of 

 high activity after many years of passage through inbred animals. It would 

 appear that the strain most suitable for exploring the problems of latency and 

 masking is the one carried by wild mice. Present knowledge of the strains 

 indicates that the ability of resistant wild mice to maintain the virus may be 

 attributed to their mixed genetic constitutions, whereas, with the inbred 

 strains, the virus has attained such a high degree of activity that it is easy to 

 detect, and masking, if it ever occurs, is an exceedingly rare event. 



