For example, some studies have suggested that MMPs have, at best, a very small contribution to osteoclast bone resorption activity and that the selective MMP-9 inhibitor, TIMP-1, did not show a significant inhibitory effect on osteoclastic bone resorption [47], while other studies have shown that MMP-9 may play a key role in bone resorption caused by osteoclasts and that patients with MMP-9 genotypes, in association with their soluble protein, may have an increased risk of developing chronic periodontitis [36,45]. This evidence concerns the gene MMP9 and periodontitis.