PAH can co-occur with hereditary hemorrhagic telangiectasia (HHT), a disease characterized by arteriovenous malformations in the lung, brain, liver, skin, and mucus membranes which implicates ACVRL1 (ALK1) and ENG mutations in the pathogenesis of PAH24–26. The gene discussed is ENG; the disease is pulmonary arterial hypertension.