MMP9 and chronic obstructive pulmonary disease: The median level of MMP‐9 transcripts appeared to be higher in the COPD‐S group than in the healthy‐S group (Fig. 5B); in contrast, the two groups did not differ significantly in terms of NGAL, MMP‐2, MMP‐12 and IL‐6 expression levels (see Fig. 5B for NGAL and Fig. S4 for MMP‐2, MMP‐12 and IL‐6).