The first mutation to be discovered in familial pulmonary aerial hypeension was in the gene encoding bone morphogenetic protein receptor type 2 (BMPR2). Inactivating germline mutations in the BMPR2 gene are found in 75% of the familial cases of pulmonary hypeension and 25% of sporadic cases. BMPR2 is a cell surface protein belonging to the TGF-b receptor superfamily, which binds a variety of cytokines, including TGF-b, bone morphogenetic protein (BMP), activin, and inhibin. Haploinsufficiency for BMPR2 leads to dysfunction and proliferation of endothelial cells and vascular smooth muscle cells.