From the viewpoint of pharmacogenomics, the antidepressant response in patients with major depressive disorder (MDD) was significantly predicted by candidate gene polymorphisms (single nucleotide polymorphism: SNPs), such as those in catecholamine-related enzymes or receptors, neurotrophic factors, and transcription factors (e.g., COMT; 5HTR2A; BDNF; CREB1), but was not associated with CYP genes (CYP1A2; CYP2C9; CYP2C19; CYP2D6) [26]. Here, CYP2D6 is linked to major depressive disorder.