This suggests that these myeloid cells are a predominant cell type that secretes MMP9 into the MMTV-PyMT metastatic niche, although other cells, including myeloid, stromal, and/or tumor cells, may also be a source of MMP9 (Psaila & Lyden, 2009) and contribute to metastatic colonization. Here, MMP9 is linked to neoplasm.