TPH2 and attention deficit-hyperactivity disorder: Even though both, genetic variants and DNA methylation are being studied in an (epi)genome-wide approach nowadays [15, 35], and large genome-wide association studies have revealed only modest effects of single genetic variation for ADHD [36], we use the candidate gene approach in this study to dedicate all analyses to the mechanism of how (epi)genetic variation effects brain/behavior relation, i.e. to link (epi)genetic TPH2 G-703T variation to the ADHD/WI phenotype in a fine-grained manner.