Endogenous mouse mammary tumor virus is expressed in several organs in addition to the lactating mammary gland.

We studied the transcription of endogenous mouse mammary tumor virus (MMTV) in transgenic and normal strains of mice of different genetic backgrounds. Although the lactating mammary gland in all strains showed the highest level of MMTV expression, detectable levels of viral RNA were also found in the lungs, kidneys, salivary glands, seminal vesicles and/or prostate gland, testes, and lymphoid tissue in mice which contain different endogenous proviruses. Transcription in transgenic mice containing the MMTV long terminal repeat linked to the simian virus 40 early region also occurred in these same organs. These results indicate that the MMTV long terminal repeat functions in several organs in addition to the lactating mammary gland.