Examination of mRNA levels in both AF and NP cells revealed that miR‐26a‐5p mimic suppressed the expression of Smad1 significantly, and its inhibitor enhanced the expression of Smad1, suggesting that miR‐26a‐5p is a bona fide endogenous inhibitor of Smad1 (Figure 5C,D). Here, SMAD1 is linked to atrial fibrillation.