SLC6A4 and pulmonary arterial hypertension: Serotonin came to the attention of the PH community in the 1960s due toan epidemic of anorexigen-associated PAH (APAH),17 and elevated levels were subsequently observed in other PAH subgroups.23 Serotonin is metabolised in the liver and lungs and transportedvia the serotonin transporter (SERT), which is present in various celltypes including pulmonary artery smooth muscle cells (PASMCs) andplatelets.