MMP13 and breast cancer: However, in the aggressive mouse mammary tumor virus-polyoma middle T-antigen (MMTV-PyMT) model of breast cancer, the absence of MMP13 did not influence tumor growth, vascularization, or metastasis to the lungs, suggesting that the role of MMP13 in breast cancer may depend on the nature of the genetic lesions driving malignancy (Nielsen et al., 2008).