MMP9 and breast cancer: MMP9 is expressed in CD11b+Gr1+ myeloid cells localized in the lungs of a mammary tumor virus (MMTV) promoter-driven polyoma middle T antigen (PyMT) mouse model of breast cancer, and MMP9 activity plays a role in the initiation and maintenance of the metastatic niche in the lungs of tumor-bearing mice [37].