Genes responsible for hereditary hemorrhagic telangiectasia (HHT; Osler–Weber–Rendu disease) are also identified as PAH-predisposing genes: activin A receptor type II-like kinase 1 (ACVRL1), endoglin (ENG), mothers against decapentaplegic, drosophila, homolog of 4 (SMAD4), and those belonging to the BMP/transforming growth factor-beta (TGF-β) super family [64]. The gene discussed is SMAD4; the disease is pulmonary arterial hypertension.