Compared with the control group, the expression level of miR-15a-5p in AF was increased significantly (Fig. 1A), and the expression level of TGF-β1, a key regulator of fibrosis, was up-regulated while Smad7 was down-regulated (Figs. 1B, 1C, 1D). Here, SMAD7 is linked to atrial fibrillation.