SLC6A4 and attention deficit-hyperactivity disorder: By examining well studied genes (including BDNF, COMT, FKBP5, NR3C1, SLC6A4, and DRD2) implicated in psychiatry disorders, including major depressive disorder (MDD), bipolar disorder (BP), schizophrenia, post-traumatic stress disorder (PTSD), attention deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), obsessive compulsive disorder (OCD), alcohol use disorder (AUD), and substance use disorder (SUD), and how their expression is regulated epigenetically, we gain insight into the molecular pathways linking lifetime experiences to lifelong mental health outcomes.