MMP9 and neoplasm: Despite the increased frequency of angiogenic switching, the vasculature within the resultant angiogenic lesions did not significantly differ phenotypically between RIP-Tag2; Arf+/+ and Arf−/− mice, consistent with previous studies in this model wherein altered angiogenic switching frequencies occurred without concomitant changes in vessel morphology or tumor cell apoptosis/proliferation upon genetic knockout of MMP-9 [33] or neutrophil ablation [24].