SLC6A3 and major depressive disorder: Although the results are not completely consistent, several studies have found that several DA-related genetic variants including catechol-O-methyltransferase (COMT), monoamine oxidase A (MAOA), DA D2 receptor (DRD2), and DA transporter gene (DAT) are closely associated with depression and antidepressant responses (Baune et al., 2008; Dong et al., 2009; Opmeer et al., 2010; Xu et al., 2011; Dannlowski et al., 2013).