SMAD9 and pulmonary arterial hypertension: Mutations of genes BMPR2 (bone morphogenetic protein receptor type 2) [8], ACVRL1 (activin A receptor like type 1) [9], ENG (endoglin) [10], SMAD9 (SMAD family member 9) [11], CAV1 (caveolin 1) [12], and KCNK3 (potassium two-pore domain channel subfamily K member 3) [13] are well-documented causes of PAH.