Hypothetical interaction between the pathways regulated by the bone morphogenetic protein receptor ligands and the serotonin (5-HT) transporter. Here, BMP receptor ligands are proposed to suppress transactivation of genes associated with smooth muscle proliferation by maintaining transcription factors (TF) bound to Smad proteins. Activation of the 5-HT transporter is normally antagonized by this pathway unless BMP receptor ligands are mutated or there is a polymorphism of 5-HT allowing for overexpression.