From the evidence generated, it is indicated that blocking the HDAC-9 pathway may be the best candidate to reduce diseases related to smooth muscle phenotypic switching within the vasculature including vein graft disease, atherosclerosis, abdominal aortic aneurysm, and pulmonary arterial hypertension, however, future studies are needed to verify this point of view. Here, HDAC9 is linked to pulmonary arterial hypertension.