In prior studies, up-regulation of microRNA-21 and downregulation of microRNAs 26, 29b, 30, 133, and 590 in atrial tissues has been associated with increased levels of fibrosis mediators such as TGF-β1 and TGFR-2 and histological evidence of left atrial extracellular fibrosis [34,35]. This evidence concerns the gene TGFB1 and fibrosis.