ACVRL1 and hereditary hemorrhagic telangiectasia: HHT1 (MIM #187300) and HHT2 (MIM #600376) that account for more than 80% of HHT are caused by heterozygous mutations in endoglin (ENG) [6-8] and activin receptor-like kinase 1 (ACVRL1; ALK1) [9-11] genes, respectively [12,13].