MMP9 and neoplasm: To evaluate the role of tumor cell-produced MMP9 in tumor progression and metastasis in vivo, we implemented an orthotopic xenograft model in which MDA-MB-231 cells stably expressing firefly luciferase (MDA-MB-231-luc2, Caliper Life Sciences) were implanted into the mammary fat pad of immunocompromised Nod/Scid mice.