SMAD1 and pulmonary arterial hypertension: At the molecular level, most cases of heritable pulmonary arterial hypertension involve mutations that affect the bone morphogenic protein (BMP) branch of the TGF signalling pathway including ACVRL1(ALK1), BMPRII, ENG, SMAD1, SMAD4, and SMAD9. Furthermore, BMPRII siRNA increases the expression of SNAI2 (Hopper et al., 2016).