Meta-analyses suggest small positive associations between the polymorphism in the serotonin transporter promoter region (5-HTTLPR) & bipolar disorder, suicidal behaviour, & depression-related personality traits but not yet to major depressive disorder. This article reviews the genetic epidemiological rationale for these studies, genetic association studies of candidate genes, genetic linkage studies of major depressive disorder & of related traits, & alternative research strategies. (130 refs.)