Bone marrow-derived progenitor cells play a critical role in vascular homeostasis and have been shown to be involved in the pathogenesis of PAH. A proliferation of c-Kit(+) hematopoietic progenitors and mast cells has been noted in the remodeled vessels in PAH. Imatinib, a tyrosine kinase inhibitor that targets c-Kit, has been shown to be beneficial for patients with PAH. Here we hypothesize that the clinical benefit of imatinib in PAH could be related to c-Kit inhibition of progenitor cell mobilization and maturation into mast cell