Single nucleotide polymorphisms (SNPs) were genotyped in the ADHD group with the Illumina (San Diego, CA, USA) Human660W-Quad BeadChip and in the control group with the HumanHap550 BeadChip. BeadStudio (version 2.0) was used to call genotypes, normalise the signal intensity data, and establish the log R ratio and B allele frequency at every SNP according to the standard Illumina protocols. All samples passed a standard SNP-based quality control procedure; all samples with a SNP call rate lower than 0·95, any duplicate or related samples (proportion identical by descent [IBD] >0·03), or any potentially contaminated samples (proportion IBD >0·03 with multiple samples) were excluded.
