HDAC9 and idiopathic pulmonary arterial hypertension: Previous studies have reported increased HDAC1 and HDAC5 expression in remodeled vessels of patients with idiopathic PAH (IPAH) and demonstrated that HDAC inhibitors could reverse hypoxia-induced PAH in rat models and in hPASMCs derived from IPAH patients by exerting antiproliferative and anti-inflammatory effects [7–9].