MMP12 and chronic obstructive pulmonary disease: The risk of COPD was 6-fold higher for heavy smokers with the T/T genotype of MMP12 than light smokers with the non T/T genotype (cOR = 6.21; 95% CI, 2.81–13.72, p<0.001, RERI = 0.671; AP = 0.108, S = 1.148).