Inhibition of MMP-9 produced predominantly by inflammatory cells in the MMTV-PyVT mouse mammary tumor model reduced lung metastasis without affecting primary tumor growth; however, this effect was only observed in mice with a genetic background derived from C57BL/6, suggesting that the pro-metastatic role of host-derived stromal MMP-9 is dependent on genetic background (82). This evidence concerns the gene MMP9 and breast cancer.