SMAD9 and pulmonary arterial hypertension: PAH has been linked with gene mutations in the BMP signal transduction pathway (e.g., ACVRL1(ALK1), BMPR1B (ALK6), ENG (endoglin), and SMAD9), but also gene mutations in genes outside of the BMP pathway have been found among those patients with and without familial forms of PAH (i.e., CAV1, KCNK3, EIF2AK4).