ACVRL1 and hereditary hemorrhagic telangiectasia: The majority of PAVMs occur as part of hereditary hemorrhagic telangiectasia (HHT) [1], [4], [11], [13], [18], [19], [20], [21] This vascular condition [22], [23], [24] is usually caused by mutations in endoglin (ENG, HHT type 1), [25]ACVRL1/ALK1 (HHT type 2), [26] or Smad4 (HHT-juvenile polyposis).[27] PAVMs affect approximately 50% of HHT patients [21] and are particularly common in HHT1, with 85% of ENG mutation carriers demonstrating right-to-left shunts on contrast echocardiography.