MMP9 and chronic obstructive pulmonary disease: According to source of controls, signifiant association of MMP-9 -1562 C/T (additive model: T vs C; OR:1.71, 95% CI: 1.42–2.07; p<0.00001, and dominant model; OR: 1.92; 95% CI: 1.34–2.76; p = 0.0004) with COPD susceptibility was revealed in the subgroup with smoker-based controls.