For example, certain authors have suggested that MMPs make, at best, a very small contribution to the bone-resorptive activity of osteoclasts [51] and that the selective inhibitor of MMP-9, TIMP-1, did not show a significant inhibitory effect on osteoclastic bone resorption [52], while other studies have reported that MMP-9 might play an essential role in the bone resorption caused by osteoclasts [53] and that patients with MMP-9 genotypes, in association with their soluble protein, may have an increased risk of developing chronic periodontitis [54]. This evidence concerns the gene MMP9 and chronic periodontitis.