252 C. p. RHOADS 



and most illuminating studies on the factors which bear on the develop- 

 ment of mammary cancer in mice. Six lines of animals were used: 



1. Group A stock lacking hormone influence. These mice are of low 

 cancer incidence in virgins and high in breeding females. They carry 

 the milk factor as shown by the occurrence of tumors in breeders and 

 transmit it to their young as shown by a 92.5% incidence in nursed Fi 

 virgins. They clearly have the factor of susceptibility, or there would 

 be no tumors even with the combination of forced breeding and milk 

 influence as shown by the C57 Blacks. The lack of hormone influence 

 is proven by a 3.9% incidence in virgins without the hormone influence 

 provided by breeding. Multiple genetic factors appear to be required to 

 produce the inherited sensitivity to or power to develop the hormone in- 

 fluence, 



2. C57 Black stock of low incidence, lacking inherited susceptibility. 

 These animals even when provided with milk factor by foster nursing 

 by A strain females and with hormonal stimulation through forced 

 breeding show only 10.3 % of cancer. The absence of inherited suscepti- 

 bility is apparent. Data obtained from crossing the A stock with the 

 C57 Black animals indicate that the inherited susceptibility is deter- 

 mined by multiple genetic factors, one of which is the gene for brown 

 coat color. 



3. Zb and Ax animals of low incidence due to lack of milk factor. 

 These are mice of the high incidence A and Z strains fostered by CBA 

 mothers lacking the factor. Both the inherited susceptibility and hor- 

 mone influence are shown by a high incidence when the animals are 

 supplied milk factor and force bred. 



4. Ax strain virgins of low incidence lacking both hormone stimula- 

 tion and milk influence. If given the latter only, as by nursing their own 

 mothers, they are, of course, the mice of Group i, still not liable to 

 cancer until provided with hormone influence by forced breeding. 



5. Z strain virgins of high incidence due to the possession of all three 

 factors. AZFi hybrids also fall in this group. 



6. Fi hybrids from crossing the A strain lacking hormone influence 

 and Z strain possessing it. A high incidence strain even in the virgin 

 females. 



From the data presented in these experiments the fact is apparent 

 that three factors operate in causing breast cancer in mice and any one 

 of the three can be completely determining in its effects. Certainly two 

 of these factors are completely controlled by genes, the inherited sus- 

 ceptibility and the inherited hormone influence. Since from Green's data 



