DRD2 and attention deficit-hyperactivity disorder: ADHD gene expression heatmap revealed higher relative expression levels across several brain tissues for the following genes; ADRA2C, COMT, dopamine receptor D1 (DRD1), DRD2, MAOA, MAOB, synaptosome associated protein 25 (SNAP25), and tyrosine hydroxylase (TH), suggesting that these genes may be considered as a risk factor for ADHD.