MMP9 and head and neck squamous cell carcinoma: While the oral cavity SCID mouse model of HNSCC requires less than 7 days to obtain measurable tumors, providing limited time for in growth of surrounding tissue, it is possible that the elaboration of secreted MMPs by surrounding tissues and the presence of MMP-2 and MMP-9 in the serum prevent formation of a tumor microenvironment devoid of MMP-2 or MMP-9.