322 BIOLOGY OF THE LABORATORY MOUSE 



nursed by females of low tumor strains. The fostered females and 

 their progeny were later used as breeders. The breast tumor incidence 

 in such fostered mice was very low (4, 6-8, 12-18). Similar results 

 have been obtained when females of other high tumor strains were fostered 

 (2, 18). 



In later work it was determined that the time interval between birth 

 and the transfer of the young to the foster mother was very important (13). 

 If the young are permitted to nurse their high tumor mother for twenty- 

 four hours or longer there is no reduction in the breast tumor incidence. 

 Progeny of these mice were not observed. 



In inbred strains of mice showing a high breast tumor incidence the 

 ratio of this type of cancer is similar among the progeny of the non-cancerous 

 and the cancerous mothers (30, 9). If fostered high tumor females develop 

 breast cancer, the incidence for the first generation progeny is comparable 

 to the control group. With each succeeding generation of progeny there is 

 a decrease in the breast tumor incidence. If the progeny of tested non- 

 breast cancerous fostered females develop mammary cancer, the tendency 

 is not transmitted. An increase in the breast tumor percentage may be 

 obtained by giving the progeny to females of high tumor strains during 

 the nursing period (13). 



No significant increase in the breast tumor incidence may be obtained 

 by fostering the young of resistant strain females to high cancer mothers 

 (6, 2, 17). Sub-line differences may account for the variations which 

 have been noticed (3, i, 17). 



The breast tumor incidence in virgin females of high tumor strains 

 depends on the stock. Some stocks have a high virgin incidence (25) while 

 others are very low (11). The foster nursing of young from all types of 

 high tumor strains, tested thus far, resulted in a reduced tumor percentage 

 for females which were used as breeders (3, 18, 2). 



If mice of low breast tumor strains are crossed to representatives of 

 high breast tumor stocks, first generation females, used as breeders, which 

 were nursed by females from the high breast tumor line showed a high 

 incidence regardless of the maternal parent. Low ratios were observed in 

 hybrids which had low tumor strain maternal parent and were not fostered, 

 or high tumor maternal parent and were nursed by low tumor strain females. 

 The evidence secured in the reciprocal first and second generation mice is in 

 accord with the theory that breast cancer susceptibility is transmitted as a 

 dominant (8, 17). Foster nursing has no apparent effect on lung cancer 

 development (16). 



