The aims of the current study are to examine gene-environment interactions associated with norepinephrine (NE) system genes (ADRA2A, SLC6A2) in ADHD, and to evaluate whether genetic changes in norepinephrine pathway are associated with differences in functional connectivity of white matter fiber tracts, as measured by diffusion tensor imaging (DTI). Furthermore, this study aims to examine neurobiological markers, such as intermediate neuroimaging phenotypes or neuropsychological endophenotypes associated with the pathophysiology of ADHD. Through evaluating drug responses and side effects with the associated measures of clinical, neuropsychological and neuroimaging characteristics, investigators would like to investigate predictors of treatment response associated with NE system genes in ADHD.