The 5-HTT gene has been associated with both depression and autism.
The serotonin transporter protein (5-HTT/SLC6A4) transports the neurotransmitter serotonin from synapses to presynaptic neurons. It seems to be an important component in the physiological response to cocaine and amphetamines. It is expressed in the central and peripheral nervous systems. The gene has long and short variants, which are identifiable by the insertion or deletion of 44 base pairs close to the beginning of the gene’s transcription site. A study by Caspi and colleagues (2003) found that individuals with the shorter variant were more likely to become depressed following environmental stress. Additionally, Kaufman and colleagues (2006) found a three-way interaction between the presence of the allele, a BDNF polymorphism, and stress. However, recent large-scale studies have failed to replicate these findings. Autistic individuals also show elevated platelet serotonin levels, and have been shown to respond to selective serotonin reuptake inhibitor (SSRI) medications.