268 PHYSIOLOGIC GENETICS 



This raises the question as to whether the level of homozygosity in an individual affects 

 tumor incidence. 



Dr. Burdette : Dr. Lynch, do you have a comment ? 



Dr. Lynch: I enjoyed hearing Dr. Heston's survey of this subject to which he him- 

 self has contributed a very great deal. The production of strains of mice has entered 

 the modern age. For example, I have been trying to produce a new strain of leukemic 

 mice, one quite different from any used so far. After a good many generations of in- 

 breeding during which with but one exception either one or both parents had the disease, 

 the leukemia was lost rather suddenly. Whether it was due to the loss of a virus or to 

 a genetic change affecting a virus or the host, I do not know. We are trying to intro- 

 duce a virus into the strain to see whether that will be effective in bringing back the 

 high incidence of leukemia. With respect to the production of some of the tumor 

 strains, we are in a viral age. 



Dr. Burdette: Historically, the two most controversial, opposing ideas of carcino- 

 genesis have been the somatic-mutation and viral hypotheses. In recent years, 

 differences between them have not seemed quite so great as formerly with the advent 

 of more information regarding cellular and viral DNA and RNA. Would you elabor- 

 ate on how what is known about the similarities between viruses and genes may be 

 related to carcinogenesis at the cellular level ? 



Dr. Heston : There are new and exciting things in this area that may have some 

 bearing on the problem of cancer. It has been interesting to consider both lysogeny 

 and transduction, although I think that some of the investigators of cancer may have 

 used these terms rather loosely. Lysogeny may be involved in the induction of tumors 

 if it can be shown that the cancer virus does enter the cell and become an integral part 

 of the genetic mechanism of the cell and in so doing causes the cell to become malignant. 

 This would be similar to lysogeny as described for bacteriophage. Such a process 

 would not be outside the limits of the somatic-mutation hypothesis of cancer if we 

 include in this hypothesis any change in the hereditary mechanism. of the cell. A 

 situation similar to transduction might be involved in the spread of cancer if, after the 

 virus has entered the cell and caused it to become malignant, some of this information 

 is then carried over to neighboring cells, causing them in turn to become malignant. 

 Such things are interesting to think about, and they are stimulating (and I hope they 

 will continue to stimulate) good research. 



