HHT has three subtypes: type 1 is caused by ENG mutations encoding endoglin, a TGF-beta co-receptor frequently involving pulmonary and neurologic AVMs; type 2 results from ACVRL1 mutations often with pulmonary hypertension and hepatic involvement; these two account for ∼85% of clinically suspected and genetically confirmed cases. The gene discussed is ACVRL1; the disease is pulmonary arterial hypertension.