MMP9 and chronic obstructive pulmonary disease: Whilst important findings on anti-protease imbalance in COPD have emerged from a number of small selective patient studies, showing negative associations between Matrix MetalloProteinase-9 (MMP-9) and lung function [6,7], such study populations can hardly be considered representative for COPD in the general population; hence, it can be questioned to what extent the observed results are generalizable.