MMP9 and breast cancer: To study the effects of MMP-9 inhibition on tumorigenesis in the context of a fully functional immune system, we used the HC11-NeuT syngeneic orthotopic mouse model of breast cancer.[33] IHC assessment of MMP-9 expression in established NeuT tumors revealed that, in contrast to the broad MMP-9 staining observed in human tumors, mouse MMP-9 expression was restricted to neutrophils in areas of necrosis (Fige 1Bi) and to macrophages in the tumor capsule region (Fig 1Bii).