HDAC9 and idiopathic pulmonary arterial hypertension: With regards to the expression profile of HDAC isoforms in PH, few studies reported elevated class I HDAC catalytic activity that correlated with increased abundance of HDAC1, HDAC2, and HDAC3 protein levels in bovine PH-fibroblasts11 and in lung tissue homogenates from hypoxia-induced PH rats and IPAH patients12.