MMP9 and atrial fibrillation: The dynamic balance of MMP-9 and TIMP-1 determines the fibrosis signal strength [21], and immunohistochemical and double-immunofluorescence staining for TIMP-1 in AF and SR tissues in the atrial interstitium showed that the increased levels of TIMP-1staining in the atrial interstitium reached statistical significance in the AF group when compared with the SR group [22].