HTR6 and major depressive disorder: In addition, there were also genes related to the serotonin neurotransmitter system and cell transport system, such as HTR2A, HTR6, TPH1, SLC1A2, SLC6A3, and SLC6A4. At the same time, the gene set included some specific genes related to MDD, such as ADCY9, ITPR1, and PCLO, which were involved in calcium signaling, binding, and salivary secretion biological pathways.