The atypical antipsychotic quetiapine has been found to have significant antidepressant action in addition to antipsychotic action. Unlike citalopram a selective serotonin reuptake inhibitor (SSRI) quetiapine has no effect on serotonin transporter (5-HTT) but it increases post synaptic 5-HT1A binding potentials and down regulates 5-HT2A receptors consistently (Tarazi et al., 2002) due to its direct antagonistic effect on 5-HT2A receptors and agonistic effect on post synaptic 5-HT1A receptors. The quetiapine induced down regulation of 5-HT2A receptors and increased density of 5-HT1A receptors has been attributed to its antidepressant effects (Thase et al., 2006). Taking into account that contrary to SSRIs, quetiapine mediated antidepressant effect does not depend on 5-HTT inhibition, it is possible that genetic factors which regulate 5-HTT expression may have less influence on the outcome of quetiapine treatment than on SSRI treatment in depressed patients.