While different serotonin receptors have been identified as critical mediators of metabolic changes in several organs (reviewed in [42]), the serotonin transporter SERT is also emerging as a pivotal player in the regulation of metabolism mediated by serotonin signalling, and SERT reduction or absence associates with metabolic syndrome and obesity [43–46]. Here, SLC6A4 is linked to obesity due to melanocortin 4 receptor deficiency.