Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin (5-HT) and might be related to the pathogenesis of major depression (MD). Two isoforms are known, TPH1 and TPH2. (1). TPH1 is phosphorylated by cAMP-dependent protein kinase A (2). Recently, it has been shown that expression of the TPH1 gene is increased in lungs and pulmonary endothelial cells from patients with idiopathic pulmonary arterial hypertension. Results indicate that TPH1 and peripheral serotonin play an essential role in the development of hypoxia-induced elevations in pulmonary pressures and hypoxia-induced pulmonary vascular remodeling. In addition, the results suggest that, in mice, serotonin has differential effects on the pulmonary vasculature and right ventricular hypertrophy (3).