Serotonin has been implicated in the pathogenesis of pulmonary arterial hypertension (PAH)1–3 and has been recognized as a potent naturally occurring pulmonary vasoconstrictor4 and smooth muscle cell mitogen.2 Serotonin promotes pulmonary artery (PA) remodeling and proliferation of human PA smooth muscle cells (hPASMCs) via the 5-HT1B receptor (5-HT1BR) and the serotonin transporter (SERT).5–8 Serotonin can also cause constriction of human and rodent PAs via the 5-HT1BR.4,9. This evidence concerns the gene SLC6A4 and pulmonary arterial hypertension.