 The investigators hypothesize that bone marrow progenitor cells are mobilized into the circulation in PAH, home to the lungs and differentiate into mast cells, which promote vascular remodeling and vasoconstriction through release of renin and chymase. As a corollary to this, the investigators hypothesize that anti cKit tyrosine kinase inhibitor TKI, nilotinib, provides clinical benefit to patients through inhibition of mast cell progenitor proliferation, mobilization and differentiation. To test this, the investigators will determine if mast cell progenitors and mast cell biomarkers are related to nilotinib clinical response. This will be an ancillary study, part of a placebo-controlled, double-blind multi center clinical trial of nilotinib in pulmonary arterial hypertension. 