The serotonin transporter (SLC6A4; 5-HTT) is another direct target of stimulant drugs, and has been associated with numerous psychiatric phenotypes including obsessive-compulsive disorder (Bloch et al. 2008), autism (Huang and Santangelo 2008), and depression (Brown and Harris 2008; Kato and Serretti 2008; Risch et al. 2009). The 5-hydroxytryptamine transporter gene-linked polymorphic region (5-HTTLPR), which may be the most widely studied polymorphism in all of psychiatric genetics, has at least two common alleles: a short (S) 14-repeat and a long (L) 16-repeat allele (Nakamura et al. 2000; Rausch 2005). The 5-HTTLPR S allele has been associated with reduced gene expression (Hranilovic et al. 2004) and increased risk for psychiatric phenotypes like depression. The long allele has been further refined into two alleles (LA and LG) distinguished by an A→G polymorphism within the first repeat; the LG allele is reported to have equivalent expression to the S allele (Hu et al. 2006). In additional to 5-HTTLPR, a VNTR in Intron 2 of SLC6A4 has also been described, which consists of either a 10- or 12-repeat allele. The 12-repeat allele has been associated with increased gene expression (Hranilovic et al. 2004).
