Pulmonary hypertension is associated with increased plasma 5-HT [32], upregulated 5-HT1B and 5-HT2B receptors [33–35] and 5-HT transporter [36–38], and enhanced 5-HT-induced pulmonary vasoconstriction [33, 36, 39–41]. This evidence concerns the gene SLC6A4 and pulmonary arterial hypertension.