Mammary tumors from BALB/c mice with a reported high mammary tumor incidence have acquired new mammary tumor virus DNA sequences

Various laboratories have reported differences in the mammary tumor incidence caused by the endogenous mouse mammary tumor virus (MMTV) in BALB/c mice. In order to resolve these differences, we have compared the MMTV specific nucleic acids extracted from BALB/c normal organs and tumor tissue obtained from laboratories reporting either a high or low BALB/c mammary tumor incidence. Hybridization kinetics and restriction endonuclease analysis indicate that mammary tumor tissue from laboratories reporting a high mammary tumor incidence contains integrated MMTV-specific DNA that is not found in normal organs from these mice and is therefore not in the germ line. Furthermore we cannot detect these acquired MMTV DNA sequences in a BALB/c mammary tumor from a laboratory reporting a low mammary tumor incidence.