PROBLEMS CONCERNING THE TUMOR VIRUSES 321 



b. Mashing. The "masking" of tumor viruses is, at the moment, very 

 controversial. The idea that a tumor virus can reside in, or combine with, the 

 host cell in such a manner that it is not detectable by known techniques has 

 been advanced for many years. Indeed, no reference is given in support of the 

 statement because it could be a separate problem of the tumor viruses to 

 give priority to the first investigator who expressed the idea in writing. 

 Anyone familiar with virologists interested in the problems of cancer has 

 heard this and other equally interesting theories expressed on numerous 

 occasions. The term "masking" has been challenged recently (Beard, 1956) 

 on the basis that the phenomenon is dependent upon the presence of a small 

 number of virus particles and not upon a difference in the state of the virus. 

 The controversy may be a matter of semantics. Perhaps a virus responsible 

 for the naturally occurring disease should be used to test the idea instead of a 

 "laboratory trained" virus, or possibly the electron microscope may not 

 differentiate between living and dead virus particles. Regardless of the out- 

 come of the controversy, a number of investigators consider seriously the 

 possibility that a virus can enter into a cell, combine with cellular components, 

 and thereby stimulate cell proliferation but not undergo replication to produce 

 new and infectious virus particles. With this understanding of the term 

 "masked" the investigator is faced with the problem of learning either how to 

 expose the hidden virus or to develop reliable techniques for the detection of 

 cells altered by the virus. 



It has been known for years that, at times, filtrates prepared from Rous 

 sarcomas do not contain virus activity. During such periods, the implantation 

 of living tumor cells is necessary for transmission of the tumor to new hosts 

 (Gye and Andrewes, 1926) and this has been interpreted (Duran-Reynals, 

 1953) as the effect of a temporary masking of the virus. Bryan et al. (1955), in 

 the discussion of their work with the initiating dose of Rous virus, review the 

 earlier observations and suggest that small quantities of virus in the inocula, 

 together with the use of resistant chickens, could account for the absence of 

 detectable virus in the filtrates. Again we are confronted with the lack of 

 quantitative studies in early investigations with this virus, and perhaps the 

 use of resistant chickens, to supply an answer to the question whether the 

 highly infectious Rous virus can become masked for any appreciable period 

 of time. Indeed, one of the basic problems of the Rous sarcoma virus is to 

 determine whether, in contrast to the tumors induced with chemical carcino- 

 gens, the proliferative power of the cells of this tumor is dependent upon the 

 presence of the virus in them. Calnan et al. (1957) attacked this problem 

 through the use of a tumor initiated with a minimum effective dose of virus 

 (Bryan et al. (1955) and on the basis of experiments showing that the rate of 

 growth of Rous sarcomas was associated with the amount of virus they 

 contained. From the original tumor three lines of transplantable tumors 

 vol. in — 21 



