SLC6A4 and major depressive disorder: Human lymphocyte SERT is significantly reduced in MDD patients compared with healthy controls.6, 7,11 Furthermore, the SSRI drug fluoxetine significantly increases the number of lymphocytes that express SERT.12 In addition, the non-SSRI antidepressant drug mirtazapine exhibits a similar effect on SERT expression, indicating that a complex mechanism that is not directly related to SERT inhibition but may possibly be related to lymphocyte growth inhibition may be functioning.7, 13