Previous studies have reported that miR‐21 is of key importance in CFs activation and cardiac fibrosis after myocardial infarction by targeting SMAD7.39 Herein, immunohistochemical analysis revealed that SMAD7 was significantly down‐regulated in T1DM mice, while inhibition of miR‐21 up‐regulated SMAD7 in perivascular tissue in diabetic myocardium (Figure 5A,B). The gene discussed is SMAD7; the disease is type 1 diabetes mellitus.