ACVRL1 and hereditary hemorrhagic telangiectasia: There are three primary genes associated with HHT: (1) endoglin (ENG) which encodes the endoglin protein, a non-intracellular co-receptor, (2) activin A receptor-like type 1 (ACVRL1) which encodes the activin receptor-like kinase-1 (ALK1), a serine–threonine intracellular receptor, and (3) suppressor of mothers against decapentaplegic (SMAD4) an intracellular transcription factor protein that aids transduction from the endoglin and ALK1 proteins toward gene expression.