Appreciation of the importance of biological factors in psychiatric disorders has been strongly reinforced by evidence from twin and family studies that genetic variation between individuals has a key role in the risk for these disorders. Heritability estimates for cognitive disorders, such as schizophrenia, attention deficit hyperactivity disorder (ADHD) and autism, range from 50% to 80% [2-6]. For affective disorders, such as major depression, anxiety disorders and substance abuse, estimates range from 40% to 65% [3,7,8]. However, pinpointing the actual genetic variants responsible for this heritability has proven difficult. The most successful gene-finding approach, genome-wide association (GWA), has uncovered many genetic variants for conditions such as diabetes [9], Crohn's disease [10] and atherosclerotic risk [11,12], but this method has, as yet, not been as successful for psychiatric disorders [13]. For schizophrenia and autism only a handful of genetic variants have been identified [14-16], and there are currently no confirmed genetic variants associated with ADHD and depression.
