ACVRL1 and hereditary hemorrhagic telangiectasia: Clinically, hereditary hemorrhagic telangiectasia type 1 (HHT1; OMIM: 187300) is caused by mutations of the ENG gene [4], whereas hereditary hemorrhagic telangiectasia type 2 (HHT2; OMIM: 600376) is caused by mutations of the ALK1 gene.