Because the changes in 5-HT-mediated neurotransmission and, specifically, in the functions of the 5-HT2A receptor are particularly well-established characteristics of molecular changes in the brains of MDD patients that are recapitulated in mouse models of depression [70,72,73,77,78], here, we studied the gene expression of 5-Htt and 5-Ht2A receptors. Here, SLC6A4 is linked to major depressive disorder.