BMPR2 and idiopathic pulmonary arterial hypertension: While most families exhibit autosomal dominant transmission, rare examples of autosomal recessive inheritance have been observed.1 2 The major genetic risk factor for both familial (ie, heritable PAH (HPAH)) and idiopathic PAH (IPAH) is haploinsufficiency of the bone morphogenetic protein receptor 2 gene (BMPR2).1 2 However, approximately 80% of IPAH cases remain genetically undiagnosed.