CXCL12 and pulmonary arterial hypertension: Furthermore, bone marrow-derived proangiogenic progenitor (CD34 + CD133 +) cells are elevated in the circulation of PAH patients [46]; interestingly, the subclinical myeloproliferative process is also present in nonaffected family members of PAH patients, and it is associated with an increase in endothelial cell production of hypoxia-inducible factor response element chemokines, such as stromal cell-derived factor 1 (SDF1) [47].