Hereditary forms of PAH (autosomal dominant with incomplete penetrance) have been identified; mutations of the following genes have been found: Activin-like kinase type 1 receptor (ALK-1) Bone morphogenetic protein receptor type 2 (BMPR2) Caveolin 1 (CAV1) Endoglin (ENG) Growth differentiation factor 2 (GDF2) Potassium channel subfamily K member 3 (KCNK3) Mothers against decapentaplegic homologue 9 (SMAD9) T-box transcription factor 4 (TBX4) Mutations in BMPR2 cause 75% of cases.