ACVRL1 and idiopathic pulmonary arterial hypertension: In different forms of pulmonary arterial hypertension (PAH) as in idiopathic (IPAH), heritable (HPAH), and PAH associated with other conditions (APAH) several mutations in genes of the transforming growth factor beta (TGF-β) superfamily of receptors such as bone morphogenetic protein receptor 2 (BMPR2) gene, Activin A receptor type II-like 1 (ACVRL1, also called ALK1) [1], Endoglin (ENG) [2], and SMAD9 and SMAD4 [3] have been found [4].