SMAD4 and pulmonary arterial hypertension: Likewise, many BMP signaling components, including BMP9, activin-like kinase 1 (ALK1), BMP receptor type II (BMPRII), endoglin (ENG) and common-mediator SMAD4, have been linked – when mutated in their respective encoding genes – to human vascular diseases, including pulmonary arterial hypertension (PAH) and hereditary hemorrhagic telangiectasia (HHT) [3].