MMP1 and atrial fibrillation: There are several limitations of this current study: first, although studies have confirmed the presence of MMP-1, MMP-2, and MMP-9 in human atrial tissue [2], these biomarkers of ECM remodelling are not confined to the myocardium and therefore we were unable to determine whether the observed relationship between MMP-9 and incident AF is due to atrial fibrosis or more generalized fibrosis.