The observation of the development of PAH in patients displaying hereditary hemorrhagic telangiectasia (HHT), an autosomal dominant vascular dysplasia, allowed us to identify two other PAH predisposing genes: ACVRL1 (Activin A receptor type II-like kinase 1) and ENG (endoglin) genes. The gene discussed is ACVRL1; the disease is hereditary hemorrhagic telangiectasia.