MMP9 and chronic obstructive pulmonary disease: In several studies including COPD populations with different clinical phenotypes, MMP-9 has been related to lower lung function [10,27,35,42] and, in our study, we have shown that the relationship between MMP-9 and lower lung function, assessed as FEV1% predicted, also applies to COPD in a population-based cohort, dominated by mild to moderate disease; GOLD stages 1–2.