We found that doxycycline improves the balance between MMP degradation and deposition by selectively inhibiting MMP-9 and MMP-2 expression on the aortic aneurysm wall, which reduces the neutrophil content on the wall, thereby reducing its inflammatory effect and preventing the growth of AAAs. We also find that doxycycline reduces the expression of MMP-9 mRNA, which further supports the anti-inflammatory effect of doxycycline. This evidence concerns the gene MMP9 and aortic aneurysm.