SLC6A3 and major depressive disorder: Folic acid, a typical ligand with a small molecule, is applied to treating brain tumors by binding specifically to the highly expressed folate receptor in the tumor cell membrane.229–231 In addition, mazindol232 and sertraline233 bind specifically to the dopamine transporter (DAT) and the serotonin transporter (SERT) on the cell membrane surface of dopaminergic neurons and 5-hydroxytryptamine (5-HT) neurons, respectively, thereby being employed in PD and depression, respectively.