336 H. B. ANDERVONT 



virus from a benign tumor virus to a virus which was highly infectious and 

 produced a fatal disease. The implication of this basic observation is that any 

 virus which, under ordinary conditions, elicits a mild degree of cellular 

 proliferation, may, when exposed to a suitable environment, be transformed 

 to an active tumor virus. Smith (1952) has reviewed the literature stemming 

 from the work of Berry and Dedrick, including her own efforts, and came to 

 the conclusion: "The transformation phenomenon was troublingly irregular." 

 She analyzed the factors involved and, among others, found: "Host factors 

 are at work of which we have as yet no clear understanding." The problems 

 of the fibroma virus, as well as those of the papilloma virus, have been 

 difficult because hosts of known genetic constitutions were not available. 



Kilham (1957, 1958) has recently applied tissue culture techniques to a 

 study of the transformation of fibroma into myxoma virus and obtained 

 interesting results. He added live fibroma virus and heat-inactivated myxoma 

 virus to cultures of rabbit testes or kidneys and obtained myxoma virus from 

 the cultures. Later he found that the transformation occurred in most 

 experiments in which cultures of rabbit kidney were used and in 4 of 10 

 experiments in which cultures of monkey kidney were used. Of interest was 

 the transformation of squirrel fibroma virus to rabbit myxoma virus in 

 tissue cultures. 



Kilham's findings are important because they not only indicate a reliable 

 procedure for reproducing the transformation but they also suggest that the 

 application of tissue culture techniques to the problems of the tumor viruses 

 may enable the investigator to avoid the disadvantages encountered in the 

 use of test animals of unknown genetic composition. It is known that cells in 

 tissue cultures thrive in body fluids from heterologous animals and perhaps, 

 in the same environment, cell- virus reactions occur which are prevented 

 in vivo by the natural resistance of the host. Recent papers by Andrewes and 

 Chaproniere (1957) and Chaproniere and Andrewes (1957), who used the 

 viruses of rabbit myxoma and fibroma, contain convincing evidence that 

 viruses specific for one species do multiply in tissue cultures of other species. 



The major contribution of the rabbit fibroma virus to the tumor viruses has 

 been the important implication that viruses responsible for the production of 

 benign tumors deserve as much consideration as those responsible for malig- 

 nant growths. 



E. Mammary Cancer of Mice 



The discovery of a virus as a causative factor in the occurrence of mammary 

 cancer in inbred mice was one of the most fascinating developments in the 

 field of virology. The sequence of events leading to its discovery and sub- 

 sequent investigations with the virus have produced results of invaluable 

 importance to the tumor viruses, because the relationships between the 



