The SLC6A4 gene encodes the serotonin transporter, a membrane protein that takes up serotonin in pre-synaptic neurons. SLC6A4 is also known as SERT or 5-HTT, since serotonin is known chemically as 5-hydroxytryptamine. The serotonin system has been studied in many behavioural and physiological conditions.
A wide range of psychiatric disorders or conditions may involve serotonin processing. The main variants of the SLC6A4 gene that have been studied, however, are not SNPs - they are short tandem repeats, also known as VNTRs (variable number tandem repeats). A review of the two VNTR polymorphisms and their potential links to human behaviour is available. [PMID 17168841]
One such polymorphism is known as the 5-HTTLPR variant, where the L allele consists of a 44bp insertion as compared to the S (short or deletion) allele. Another polymorphism is the STin2 (intron 2) VNTR, which involves different alleles that correspond to 12-, 10-, 9-, or 7-repeat units of 17bp. Both of these polymorphisms have been associated in some cases (but not others) with obsessive-compulsive disorder (OCD). Most recently, the STin2.12 carriers were reported to be at over 3x risk of OCD based on a study of ~100 OCD patients.[PMID 18191318]
A few SNPs have been studied; these include: