The MMTV-PyVmT transgene has the mouse mammary tumor virus (MMTV) long terminal repeat upstream of a cDNA sequence encoding the Polyoma Virus middle T antigen (PyVmT). Transgenic mice are viable, but show loss of lactational ability due to activation of the estrogen responsive transgene during pregnancy leading to tumorigenesis. Female carriers develop palpable mammary tumors with a mean latency of 92 days of age, as compared with 53 days in FVB/N-Tg(MMTV-PyVT)634Mul/J mice (Stock No. <a href="https://www.jax.org/strain/002374">002374</a>). Adenocarcinomas arise in virgin and breeder females and occasionally in males, which are well-differentiated, multifocal and eventually involve the entire mammary fat pad. Tumor-bearing females have reduced lung metastasis compared to the 80-90% incidence found in FVB/N-Tg(MMTV-PyVT)634Mul/J mice. Transgene expression is detected at high levels in male and female mammary glands. Lower levels are detected in salivary gland, seminal vesicles, ovaries, and lungs (believed to be the result of pulmonary metastases) [<a href="https://pubmed.ncbi.nlm.nih.gov/1312220/">Guy et al., 1992, Mol Cell Biol.</a>]. This strain may be used as a platform to alter the tumor microenvironment. On the FVB/N background, tumor burden and pulmonary metastases have been shown to be reduced after in vivo TMP195 treatment by inducing the recruitment and differentiation of highly phagocytic and stimulatory macrophages within tumors. TMP195 treatment combined with chemotherapy regimens or T-cell checkpoint blockade was shown to significantly enhance the durability of tumor reduction.&nbsp;&nbsp;

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MMTV-PyMT (MMTV-PyVmT) females develop palpable mammary tumors which metastasize to the lung. Mean latency is 92 days of age.  Male carriers develop mammary tumors with a later onset.  These mice express the Polyoma Virus middle T antigen under the direction of the mouse mammary tumor virus promoter/enhancer and may be suitable for use in studies related to breast cancer and altering the tumor microenvironment.

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