ACVRL1 and hereditary hemorrhagic telangiectasia: Hereditary Hemorrhagic Telangiectasia (HHT) is an autosomal dominant disorder resulting from loss of function mutations predominantly in ENG (HHT type 1) or ACVRL1 aka ALK1 (HHT type 2) (Shovlin, 2010; Faughnan et al., 2011).