ACVRL1 and hereditary hemorrhagic telangiectasia: Three genes are causally related to hereditary hemorrhagic telangiectasia (HHT): the ENG gene encoding the co-receptor endoglin (HHT1; McAllister et al., 1994), the ACVRL1 gene encoding ALK1 (Johnson et al., 1996), and the SMAD4 gene, a critical factor in this signaling pathway (Gallione et al., 2004).