HTT and major depressive disorder: Other genetic links with depression have been reported through polymorphisms and allele dominance on the serotonin transporter gene 5-HTT, [22], the piccolo gene PCLO, which influences monoaminergic neurotransmitters in the brain [23], and gene-linked malfunctions of the glutamate and GABA systems [24], which reduce the functionality of the neurotransmitters serotonin [25] and dopamine [26], thus impairing cognitive performance, one of the symptoms of depression [27].