MMP9 and periodontitis: We suggested that several factors including genetic backgrounds for cases and controls, diverse genotype distribution of MMP-2 -753C>T and MMP-9 -1562C>T polymorphisms in the included ethnicity groups, types of periodontitis (CP/AgP), different genotyping methods, sample size of included studies, and uneven selection criteria for the cases and controls in different studies, responsible for such heterogeneity in our meta-analysis.