SLC6A4 (Solute carrier family 6, member 4), also known as SERT (serotonin transporter), is a monoamine transporter protein. SLC6A4 transports the neurotransmitter serotonin from synaptic spaces into presynaptic neurons. The transport of serotonin by the SERT protein terminates the action of serotonin and recycles it in a sodium-dependent manner. SLC6A4 is the target of many antidepressant medications, including those of the SSRI class. It is a member of the sodium:neurotransmitter symporter family. A repeat length polymorphism in the promoter of this gene has been shown to affect the rate of serotonin uptake and may play a role in sudden infant death syndrome, aggressive behavior in Alzheimer disease patients, post-traumatic stress disorder and depression-susceptibility in people experiencing emotional trauma.