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.

Inclusion Criteria:

This study is a substudy and subjects must be enrolled in the main trial:

See Study Efficacy, Safety, Tolerability and Pharmacokinetics (PK) of Nilotinib (AMN107) in Pulmonary Arterial Hypertension (PAH) In clinical trials. (clinical Trial ID NCT01179737)

http://clinicaltrials.gov/ct2/show/NCT01179737?

Exclusion Criteria:

Subjects are not enrolled in the main study:

See Study Efficacy, Safety, Tolerability and Pharmacokinetics (PK) of Nilotinib (AMN107) in Pulmonary Arterial Hypertension (PAH) In clinical trials. (clinical Trial ID NCT01179737)

http://clinicaltrials.gov/ct2/show/NCT01179737?term=tasigna+and+pulmonary+hypertension\&rank=1

Patients diagnosed with PAH that are enrolled in the Nilotinib clinical trial. http://clinicaltrials.gov/ct2/show/NCT01179737?term=tasigna+and+pulmonary+hypertension\&rank=1