HDAC9 and pulmonary arterial hypertension: Except for class III HDACs, which are known as sirtuins (SIRT1-7), the other three classes of HDACs are zinc-dependent enzymes, which are commonly targets of small chemical HDAC inhibitors.31, 32 Recent evidence has suggested that inflammation widely contributes to cardiovascular pathogenesis, including the pathogenesis of PAH.33 Importantly, treatment with HDAC inhibitors at a low dose can attenuate the inflammatory response in chronic cardiovascular conditions, providing the potential functional mechanism of HDAC inhibitors in therapeutic treatment.34