Differences in mouse mammary tumor viruses. Relationship to early and late occurring mammary tumors.

The murine model has been used extensively to study the various factors involved in the etiology of mammary carcinoma. Inbred mouse strains have been classically categorized into (i) high incidence stains with tumors occurring relatively early in the life of the animal, or (ii) low or moderate incidence strains with tumors occurring later on in life. We have radioactively labeled the RNA genome of the mouse mammary tumor virus (MMTV) from each of several mouse strains. We report here, using the technique of molecular hybridization, that the class of MMTVs responsible for the early occurring mammary tumors in high incidence strains can be clearly distinguished from the MMTVs associated with late occurring mammary tumors in low or moderate incidence strains; we also demonstrate that minor differences in MMTV genomes can also exist within these classes. Our findings show that MMTVs are transmitted via the germ line (as a germinal provirus) in some mouse strains, whereas in other strains, a non-germ line transmission is clearly demonstrated. Biochemical techniques can thus be used to track the mode of transmission of oncogenic viruses. The relationship of these findings to an understanding of the etiology of mammary carcinoma is discussed.