As shown in Fig. 4A, MCT obviously reduced miR-34a level to 0.62-fold compared with control group (P < 0.05), while treatment of MCT-induced PAH rats with class I HDAC inhibitor MS-275 restored the miR-34a expression to 0.89-fold over control (P < 0.05 versus MCT group). This evidence concerns the gene HDAC9 and pulmonary arterial hypertension.