Transfer, by selective breeding, of the pathogenic Mtv-2 endogenous provirus from the GR strain to a wild mouse line free of endogenous and exogenous mouse mammary tumor virus.

The GR laboratory mouse strain has five endogenous proviral copies of the mouse mammary tumor virus (MMTV). One of these, Mtv-2, is unique because it causes mammary carcinomas in virtually 100% of breeding GR females prior to 1 year of age. To facilitate studies of this locus in particular, and mammary tumorigenesis in general, we genetically tailored a new mouse line, WXG-2, which bears Mtv-2 as its only endogenous MMTV provirus. The WXG-2 line was constructed by making hybrids between the GR strain and a wild mouse line free of both endogenous and exogenous MMTV, backcrossing to the MMTV-free line, and fixing the Mtv-2 locus in a population with the desired genotype. Mammary tumors were observed in 5 of the 20 hybrid females carrying the endogenous Mtv-2 provirus. The WXG-2 line represents a new model system for studying MMTV-induced mammary tumorigenesis in the absence of multiple endogenous proviruses.