Using a well characterised sample, we show that children with ADHD have a significantly increased burden of large, rare CNVs that include both duplications and deletions. Importantly, since CNV burden is increased in people with unexplained intellectual disability, autism, and schizophrenia, our clinical assessment procedure allowed us to conclude that the CNV burden is not attributable to comorbidity with these disorders. Although ADHD is one of the most heritable psychiatric disorders, with a heritability estimate of 76%,1,2 genome-wide association and linkage studies have so far failed to identify common genetic risk variants. Our results suggest that further investigation of rare CNVs in ADHD is likely to be fruitful. Up to now, there has been only one reported investigation of CNVs in ADHD, which did not detect a significantly increased burden of rare CNVs in 335 patients with the disorder.12 However, that study examined CNVs of all sizes rather than large CNVs, which are the ones that are most likely to be deleterious24 and are particularly enriched in neurodevelopmental disorders such as schizophrenia.9–11
