32G H. B. ANDERVONT 



The authors interpreted these results as indicating that the particles 

 represented, or were quantitatively related to, the Rous virus. The high 

 incidences (70-95 %) of cells containing particles in tumors induced with 

 cell-free materials of high potencies suggested that the detectable particles 

 were responsible for the tumors, thereby obviating the necessity of postulating 

 a nondetectable stage in the development of the virus. Further studies with 

 closer serial dilutions of virus are needed to settle this point. 



d. Purification. The problem of purification of the Rous virus has proved to 

 be extremely difficult, but the by-products of a sustained effort by Bryan and 

 his colleagues to attain this goal have been so rewarding that the final 

 achievement will have long since contributed much to the major problems of 

 the tumor viruses. 



A recent review by Bryan and Moloney (1957) of the purification problem 

 makes unnecessary another summation. Many obstacles have been overcome. 

 Use of citrate buffers (Bryan et al., 1950; Bryan, 1955) aided much in stabiliz- 

 ing this extremely fragile virus and present studies are aimed at methods for 

 the prevention of harmful water-soluble products of fatty-acid oxidation. 

 Progress has also been made on the problem of dissociating the virus from the 

 large quantity of nonviral material in tumor extracts. 



One major problem in purification of the virus is to procure starting 

 material with a sufficiently high virus content. This appears to have been 

 essential for the purification of other tumor viruses (Beard et al., 1955) and is, 

 in all probability, the next major problem in the purification of the Rous 

 virus. The time-honored procedure of selecting "tumors of especially rapid 

 growth," as described by Rous (1911), together with the use of the most 

 potent virus suspension for each animal passage, may supply an answer to this 

 problem. Bryan and Moloney (1957) reported some success with this technique 

 and also suggested the use of tissue cultures. As stated previously, it is 

 strange that the Rous virus has escaped notice by those interested in the 

 propagation of viruses in tissue cultures. 



This summation of recent work*with the virus of Rous sarcoma is presented 

 to illustrate how this virus is now being used to elucidate cell-virus relation- 

 ships which must be of importance in the occurrence of this virus-induced 

 tumor. These efforts are, therefore, an extension of the chief contributions the 

 Rous virus has made to the tumor virus problems. It has always served as the 

 "pilot" virus for other tumor viruses and is now used to compare a tumor 

 virus with other viruses as soon as any advance is made in studies with plant, 

 bacterial, or animal diseases. It should always be kept in mind, however, that 

 results with one tumor virus may not apply to others. And this could apply 

 to the Rous virus because many years of serial passage through large numbers 

 of hosts could have done much to alter its biological properties. Rous (1910) 

 found that successful propagation of the original tumor cells was possible 



