As a result, researchers initially focused their efforts on variants in genes such as the serotonin transporter (5-HTTLPR), monoamine oxidase A (MAOA), dopamine receptor 4 (DRD4), and dopamine transporter (DAT) genes that were known to be associated with risk for various psychiatric problems, including depression, substance use disorder, and attention-deficit/hyperactivity disorder (ADHD; Caspi et al., 2003; Okuyama et al., 2000; Rowe et al., 1998). Here, SLC6A3 is linked to major depressive disorder.