Genetic analyses of differences in incidence of mammary tumors and reticulum cell neoplasms with the use of recombinant inbred lines of mice.

The influence of genes, in addition to genes in the H-2 complex, that effect the genesis of mammary tumors was studied. The recombinant inbred (RI) CXB lines were chosen for this investigation, because they are well suited for the study of the genetics of a trait for which the genotype affects probability of phenotype expression and which therefore is measured as incidence. Females of seven RI lines (CXBD, CXBE, CXBG, CXBH, CXBI, CXBJ, and CXBK) and their progenitor strains C57BL/6By (B6) and BALB/cBy (BALB/c) were given ip injections of MuMTV at 3 months of age and were force bred. They were observed for mammary tumors. The B6 strain was least susceptible, and mammary tumors appeared late in life. The BALB/c strain was most susceptible, and the tumors appeared early in life. The course of tumor development in the RI lines fell between these extremes. The RI strain distribution pattern of mammary tumor incidence indicated that at least one and probably several loci in addition to those at H-2 determined the difference between the BALB/c and B6 strains. Effects of the other gene(s) appeared to be even more important than those of H-2. The locations of those loci were not made clear by this study. The spontaneous incidence of reticulum cell neoplasms was also recorded. The most frequently formed neoplasm of the reticular system was a Hodgkin's-like lesion. The data suggested an influence of the H-2 complex.