Primary pulmonary arterial hypertension is a rare, often fatal, progressive vascular lung disease characterized by increased pulmonary vascular resistance and sustained elevation of mean pulmonary arterial pressure, leading to right ventricular hypertrophy and right heart failure. Pathologic features include a narrowing and thickening of small pulmonary vessels and plexiform lesions. There is pulmonary vascular remodeling of all layers of pulmonary arterial vessels: intimal thickening, smooth muscle cell hypertrophy or hyperplasia, adventitial fibrosis, and occluded vessels by in situ thrombosis (summary by Machado et al., 2009 and Han et al., 2013). Heterozygous mutations in the BMPR2 gene are found in nearly 70% of families with heritable PPH and in 25% of patients with sporadic disease. The disease is more common in women (female:male ratio of 1.7:1). However, the penetrance of PPH1 is incomplete: only about 10 to 20% of individuals with BMPR2 mutations develop the disease during their lifetime, suggesting that development of the disorder is triggered by other genetic or environmental factors. Patients with PPH1 are less likely to respond to acute vasodilater testing and are unlikely to benefit from treatment with calcium channel blockade (summary by Machado et al., 2009 and Han et al., 2013). Genetic Heterogeneity of Primary Pulmonary Hypertension See also PPH2 (615342), caused by mutation in the SMAD9 gene (603295) on chromosome 13q13; PPH3 (615343), caused by mutation in the CAV1 gene (601047) on chromosome 7q31; PPH4 (615344), caused by mutation in the KCNK3 gene (603220) on chromosome 2p23; PPH5 (265400), caused by mutation in the ATP13A3 gene (610232) on chromosome 3q29; and PPH6 (620777), caused by mutation in the CAPNS1 gene (114170) on chromosome 19q13. Primary pulmonary hypertension may also be found in association with hereditary hemorrhagic telangiectasia type 1 (HHT1; 187300), caused by mutation in the ENG gene (131195), and HHT2 (600376), caused by mutation in the ACVRL1 (ALK1) gene (601284). Pediatric-onset pulmonary hypertension may be seen in association with ischiocoxopodopatellar syndrome (ICPPS; 147891). The skeletal manifestations of ICPPS are highly variable and may not be detected in children. Parents are not likely to have PAH (Levy et al., 2016).