Mutations in endoglin (ENG) and activin receptor-like kinase 1 (ACVRL1) (encoding ALK1) have been linked to a human vascular disorder hereditary hemorrhagic telangiectasia (HHT1 and HHT2, respectively), often resulting in arteriovenous malformations (AVM) [75,76]. The gene discussed is ACVRL1; the disease is hereditary hemorrhagic telangiectasia.