ACVRL1 and pulmonary arterial hypertension: Variants in two other genes in the transforming growth factor-beta (TGF-β) superfamily, activin A receptor type II-like 1 (ACVRL1), and endoglin (ENG) contribute to ~ 0.8% of PAH cases [7], especially PAH associated with hereditary hemorrhagic telangiectasia (APAH-HHT).