Apoptosis is another key factor in vascular remodeling characteristic of PAH, as such, it has been previously shown that serotonin inhibits PASMC apoptosis through 5-HT1BR or SERT.38 It would be interesting to understand the role of serotonin and the 5-HT1BR in other vascular cell components, such as human PA endothelial cells, primary cultures of such cells are not readily available from control subjects and patients with PAH, and therefore are difficult to study in relation to the human disease. This evidence concerns the gene SLC6A4 and pulmonary arterial hypertension.