MMP9 and periodontitis: Although a number of MMPs have been described in gingival tissues (such as MMP-2, MMP-7, and MMP-14) [26], the most widely reported MMPs in gingival crevicular fluid (GCF) are MMP-8, MMP-9, and MMP-13 [27,28,29,30], and among the three collagenases (MMP-1, MMP-8, and MMP-13), MMP-8 accounts for 80% of the total collagenase protein found in the GCF, with smaller relative amounts of MMP-13 and MMP-1 in chronic periodontitis [31].