Farha et al. (2011) reported that nonaffected family members of patients with familial pulmonary arterial hypertension displayed elevated circulating levels of CD34+CD133+ progenitor cells, which were comparable with their affected relatives with pulmonary arterial hypertension, and had a significant increase in marrow fibrosis compared with healthy unrelated controls. Here, PROM1 is linked to pulmonary arterial hypertension.