5-HTTLPR is a repeat length polymorphism that has been shown to affect the efficiency of serotonin uptake and thus synaptic and extracellular serotonin concentrations in the brain. The short (s) allele of 5-HTTLPR which shows a reduced transcriptional efficiency compared to the long (l) variant predisposes to cognitive vulnerability to stress-sensitivity including anxiety-related personality traits such as neuroticism [3], or amygdala reactivity to aversive stimuli [4] which are risk factors for major depressive disorder (MDD) [5,6]. Collier et al. [7] suggested that there is a significant, although weak, association between depression and 5-HTTLPR s allele. Subsequently, primate studies demonstrated that the effect of the 5-HTTLPR s allele on serotonin function in the central nervous system and on behavior is modulated by early rearing conditions providing the first evidence for GxE [8,9].
