SLC6A4 and psychiatric disorder: In genetic studies of psychiatric disorders, one of the most commonly studied locus in the pre-GWAS era [6], is the serotonin transporter (5-HTT or SLC6A4) promoter polymorphism, a 43-base-pair insertion/deletion polymorphism in the promoter region (5-HTTLPR) with a long (L) and a short (S) allele.