MMP1 and pulmonary emphysema: The authors hypothesised that a switch from transitory to constitutive ERK activation may be a critical event in emphysema progression by increasing the production of matrix metalloproteinase 1, which has been causally linked to the development to emphysema.29, 30 Furthermore, while acknowledging the importance of cigarette smoke exposure in the aetiology of emphysema, the authors were able to demonstrate the increased activity of ERK in emphysema to be independent of ongoing cigarette smoke inhalation; we also failed to find an association between phosphorylated ERK expression and smoking.