Characterization and chromosomal location of endogenous mouse mammary tumor virus loci in GR, NFS, and DBA mice.

Endogenous mouse mammary tumor virus (MMTV) proviral copies were characterized in three genetically dissimilar mouse strains: GR, a high-tumor-incidence strain bred in Europe that carries an MMTV proviral copy associated with early mammary tumors; DBA, a high-tumor-incidence laboratory strain bred in the USA with an endogenous copy that is associated with MMTV antigen expression in the milk; and NFS, a recently inbred line of the low-tumor-incidence NIH Swiss mouse. MMTV proviral loci were studied using restriction endonuclease analysis and the Southern transfer procedure in genetic crosses and in somatic cell hybrids. By studying the segregation of MMTV-specific EcoRI, BamHI, and PstI fragments, the organization of these fragments into MMTV proviral loci was determined and it was shown that (1) many homologous proviral loci are present in these three mouse strains, (2) these MMTV proviruses differ in their pattern of internal restriction sites, and (3) the MMTV loci are distributed on multiple chromosomes including 1 and 7.