Moreover Platelet-derived growth factor (PDGF) is a potent smooth muscle cell mitogen that may contribute to smooth muscle hyperplasia during the development of chronic pulmonary hypertension and selective PDGF inhibition may provide a novel therapeutic strategy for the treatment of chronic PH. Pulmonary arterial hypertension, a disorder limited to the pulmonary circulation, is characterized by pulmonary vascular obstruction and variable pulmonary vasoconstriction leading to increased pulmonary vascular resistance and death. Pulmonary hypertension is a frequent complication of lung fibrosis in systemic sclerosis, but can also occur without overt fibrotic pulmonary involvement in particular in the limited form of scleroderma. The PDGF receptor antagonist STI571 (imatinib) reversed advanced pulmonary vascular disease in 2 animal models of pulmonary hypertension regardless of the initiating stimulus and it has been used successfully in single patients resistant to conventional therapy .