SMAD9 and pulmonary arterial hypertension: There are more than 10 diseases causing genetic mutations that have been linked to the development of PAH, including bone morphogenetic protein receptor (BMPR2), SMAD family member 9 (SMAD9), activin A receptor-like type 1 (ACVRL1), endoglin (ENG), and eukaryotic translation initiation factor 2 alpha kinase 4 (EIF2AK4).