Mammary tumors from GR mice contain more than one population of mouse mammary tumor virus-infected cells

Spontaneous mammary tumors in the GR mouse strain contain several acquired copies of mouse mammary tumor virus (MMTV) DNA that are not present in normal organ DNA and that are detectable by restriction endonuclease analysis and the Southern blotting procedure. Hormone-responsive and -independent cell populations were selected from spontaneous GR mammary tumors by grafting the tumors in castrated male GR mice in the presence and absence of female sex hormones. Analysis of the acquired MMTV DNA copies revealed differences between hormone-responsive and -independent cells derived from the same tumor; however, specific MMTV DNA fragments could not necessarily be correlated with the hormone responsiveness of the tumor. In some cases more than one proviral pattern could be detected for both hormone-responsive and hormone-independent cells. These results suggest that spontaneous GR mammary tumors are made up of more than one population of hormone-responsive and -independent cells which can be distinguished by their MMTV-specific proviral restriction fragments.