Candidate gene studies have previously implicated polymorphisms within the X-linked MAOA and HTR2C genes in phenotypes associated with ADHD [41, 42]; recent recognition that the X chromosome must also be incorporated in genomewide association studies together with the development of statistical tools to perform such analyses (e.g. XWAS [43]), is likely to drive the generation unbiased evidence for association between multiple X-linked genetic variants and ADHD-related phenotypes in the near future. This evidence concerns the gene MAOA and attention deficit-hyperactivity disorder.