Since aberrant TGF-β signaling can inhibit the expression of HO-1 (Okita et al., 2013), we decided to investigate the paracrine effects of TGF-β in endothelial HO-1 expression under chronic hypoxia (HOX, 1% O2 for 24 h; a well-established in vitro model of PAH) (Aley et al., 2008) and normoxia conditions (Control, 20% O2). The gene discussed is TGFB1; the disease is pulmonary arterial hypertension.