rate of serotonin uptake [10]. Specifically, the short (S) allele of the 5-HTTLPR is associated with less transcriptional efficiency of the promoter compared to the long (L) allele [11]. Additionally, a single nucleotide substitution (rs25531, A > G) within both alleles reduces transcription so that the LG allele becomes functionally equivalent to the S allele [12,13]. Studies have suggested grouping LG with the S allele to increase efficiency in predicting variation in serotonin transporter expression [14], although not all agree on this point.
