SMAD9 and pulmonary arterial hypertension: The pathogenesis of PAH has been associated with genetic factors that confer a predisposition for development or progression, such as mutations of the bone morphogenic receptor type II gene (BMPR2), a member of the transforming growth factor superfamily TGF-β (Newman et al., 2001), mutations in the activin A receptor type II 1 (ACVRL1) (Girerd et al., 2010), endoglin (ENG) (Chaouat et al., 2004), SMAD9, caveolin-1 (CAV1) and KCNK3 (Ma et al., 2013).