Among patients with heritable forms of PAH, many have germline mutations in genes encoding receptors of the transforming growth factor-beta (TGF-β) receptor superfamily, usually the bone morphogenetic protein (BMP) receptor type 2 gene (BMPR2) and less often the ACVRL1 or ENG gene encoding activin receptor-like kinase 1 (ALK1) and endoglin (ENG), respectively. Here, ACVRL1 is linked to pulmonary arterial hypertension.