PROBLEMS CONCERNING THE TUMOR VIRUSES 341 



It is essential to mention that the great majority of experimental results 

 mentioned above were carried out with inbred mice which were highly 

 susceptible to the virus and in which the virus is transmitted from generation 

 to generation. These investigations were not performed with animals of 

 unknown genetic composition and there is every reason to assume that a small 

 amount of virus would produce tumors in them and, furthermore, that in 

 successive generations the virus would increase sufficiently to produce 

 tumors. The only conclusion permissible at present is that the weight of 

 evidence favours the occurrence of mammary tumors in virus-free mice. 



If the virus is not essential for the production of all breast cancers in mice, 

 then studies with this virus have made another significant contribution to the 

 problems of the tumor viruses, for it becomes clear that the discovery of a 

 viral etiology for a tumor does not prove the virus is responsible for all other 

 tumors of the same type. Other viruses or no virus may be involved in these 

 latter tumors. This makes the search for tumor viruses more difficult because 

 they may play a minor part in the complicated interactions of various 

 influences resulting in malignancy. 



2. The Latency of the Virus 



If the term latent implies the presence of a virus in the host without the 

 production of the disease, then the mammary tumor virus of mice certainly 

 has a long latent period. A newborn mouse need remain with its mother for 

 only a few hours after birth to acquire sufficient virus for the occurrence of a 

 tumor in middle or late life. Further, during this time, while the mice are 

 without tumors, the virus is detectable in tissues of infected animals and is 

 always present in the milk. This unique property of the virus made possible 

 its discovery, for geneticists had developed inbred strains of mice with varying 

 degrees of susceptibility to the development of mammary cancer before the 

 virus was known. After the virus was found, it was learned that all high- 

 mammary cancer strains carried it whereas low-mammary cancer strains did 

 not. Selection toward susceptibility to the tumor was also selection toward 

 susceptibility to the virus; this implies that the genetic make-up of the host 

 may be of much importance in the activation of a latent virus. 



At least one inbred strain was established which did not develop tumors 

 but was susceptible to the virus; when mice of strain BALB/c were suckled by 

 foster mothers from strain C3H that carried the virus, the BALB/c strain was 

 changed to a high-tumor strain (Andervont, 1945c, 1949b). The ability of 

 strain BALB/c to propagate the virus became of more interest when it was 

 discovered, as stated previously, that hybrids derived from presumably 

 virus-free BALB/c females and strain C3H animals showed a high incidence of 

 mammary tumors (Andervont, 1945b). It was concluded that the tendency 



