There are four major types of PAH: (1) idiopathic PAH (IPAH), formerly termed primary pulmonary hypertension with unknown etiology; (2) heritable PAH (HPAH), formerly known as familial PAH, which is caused in most cases by mutations in the BMP receptor type 2 gene (BMPR2) encoding BMPR-II; (3) drug- and toxin-induced PAH; and (4) PAH associated with other conditions such as congenital heart disease, connective tissue disease, and portal hypertension [77]. This evidence concerns the gene BMPR2 and connective tissue disorder.