This study is the second independent genome-wide association study of children with ADHD and the first conducted in children from the United States. We failed to identify any statistically significant genome-wide findings, but found some additional evidence for a role of SLC9A9 in the etiology of ADHD. SLC9A9 codes a hydrogen/sodium exchanger located in the membranes of subcellular structures and is expressed in the brain and the heart and skeletal muscle, placenta, kidney and liver. 18 SLC9A9 was first identified at a break point in a family in which ADHD symptoms were correlated with a chromosome 3 inversion. 18 It was also associated with ADHD in the IMAGE study of 51 candidate genes for ADHD 3 and in subsequent quantitative analyses of ADHD symptoms and age at onset. 6, 19 In the current study SLC9A9 was the most significant candidate gene examined (p=0.0017 gene-wide) and the only candidate with a SNP (rs9810857-T, p=6.4E-6) among the top 20 significant associations in the genome-wide scan of 835,136 SNPs.
