MMP3 and periodontitis: When stratified by the type of periodontitis, we found a significant association of the -1171 5A/6A polymorphism in the MMP3 promoter region with both CP (6A allele vs. 5A allele: OR = 0.72, 95% CI = 0.63–0.82, Pheterogeneity = 0.163; 5A/6A vs. 5A/5A: OR = 0.50, 95% CI = 0.39–0.64, Pheterogeneity = 0.345; 6A/6A vs. 5A/5A: OR = 0.41, 95% CI = 0.25–0.68, Pheterogeneity = 0.336; 5A/6A + 6A/6A vs. 5A/5A: OR = 0.48, 95% CI = 0.38–0.62, Pheterogeneity = 0.317, respectively) and AgP (6A allele vs. 5A allele: OR = 0.66, 95% CI = 0.48–0.91, Pheterogeneity = 0.945) (Fig. 2).