SMAD9 and pulmonary arterial hypertension: Diverse loss-of-function mutations associated with heritable PAH have been identified in genes encoding bone morphogenetic protein (BMP) signaling molecules and their downstream effectors, including BMPR2, ACVRL1, ENG, GDF2 (encoding BMP9), and SMAD9 (SMAD8), implying that the BMP signaling branch of the superfamily exerts a protective function that is compromised in disease3,9.