SLC6A4 and major depressive disorder: Meta-analysis studies demonstrated an association between depression and response to antidepressants and the long/short forms (known as L/S alleles) of promoter repeats in the SLC6A4 gene (also known as 5-HTTLPR, 5-HTT, or serotonin transporter, SERT).22,23 Studies have identified missense variants in the coding region of SLC6A4 and intronic SNPs that affect response to SSRIs and SSRI-induced side effects.24,25 However, the relative effect size of each variant remains unknown.