The majority of HHT individuals will have HHT1 due to mutations in ENG encoding endoglin (McAllister et al., 1994) or HHT2 due to mutations in ACVRL1 encoding Activin receptor Like-Kinase 1 (ALK1) (Johnson et al., 1996). This evidence concerns the gene ACVRL1 and hereditary hemorrhagic telangiectasia.