SLC6A4 and mental disorder: In fact, functional genetic variants of serotonin transporter gene (5-HTT), which transports serotonin from the extracellular space, and the monoamine oxidase A gene (MAO-A), which is a key enzyme responsible for degrading serotonin, have been implicated in mental disorders such as mood and anxiety disorders [3], [5], [34]–[37].