BDNF and attention deficit-hyperactivity disorder: Several studies from genomics to metabolomics, as well as those including measurements of monoamine (e.g., dopamine [DA], serotonin [5-HT], and noradrenaline [NA])15, hormone (e.g., cortisol and oxytocin)16, and neurotrophic factor (e.g., brain-derived neurotrophic factor [BDNF])17 levels in the plasma, have been performed to identify potentially predictive peripheral and genetic biomarkers for ADHD and ASD18.