SLC6A4 and major depressive disorder: We believe that the most interesting results within our study were obtained for the STin2 polymorphism of the serotonin transporter gene SLC6A4. It is well documented that disturbances in the serotonin-controlled neuromodulation system involving the serotonin transporter are implicated in the pathogenesis of many behavioral disorders (anxiety, depression, obsessive compulsive disorder, aggression, addiction, suicide) as well as in major psychiatric conditions, such as bipolar disorder and schizophrenia [42, 43].