SLC6A4 and depressive disorder: These genes are involved in the complex pathological mechanism of depression, including the neurotransmitter system (e.g., HTR2A and SLC6A4), neurotrophin (e.g., BDNF), hypothalamic-pituitary-adrenal (HPA) axis (e.g., CRH), and inflammation (e.g., IL6, TNF).