Mutations of ACVRL1, the gene for ALK-1, are associated with the appearance of hereditary hemorrhagic telangiectasia type 2, a vascular dysplasia characterized by telangiectasias in the skin, nose bleeds and arteriovenous malformations in several organs, mainly in the liver (Johnson et al., 1996; Shovlin et al., 1997). The gene discussed is ACVRL1; the disease is hereditary hemorrhagic telangiectasia.