Expression of CXCL12 is markedly increased in the pulmonary vascular endothelium and the endothelium of the vasa vasorum of larger pulmonary vessels removed from patients with PH when compared to control lungs [6, 18, 19] and endothelial cells isolated from the lungs of PAH patients produce greater amounts of CXCL12 in vitro than cells from control lungs [18]. Here, CXCL12 is linked to pulmonary arterial hypertension.