Despite the fact that 5-HT is of crucial importance to the pathogenesis of depressive disorder, we did not observe any relationship between BDI, 5-HT, SERT protein, and mRNA SERT. This lack of association could result from the high subjectivity of the BDI questionnaire, which is filled out by a patient, not a clinician. Here, SLC6A4 is linked to depressive disorder.