Despite the great progress made in recent decades, our knowledge of the pathophysiology of many common neuropsychiatric disorders remains limited. The lag experienced in this domain compared with other medical fields is not without cause; several unique obstacles exist in the study of mental diseases. First, mental disorders represent dysfunction in the least understood organ of the human body: the brain. Second, our limited understanding for the genetic basis of mental illness suggests great complexity. Studies on twins repeatedly show that the genetic makeup of affected neuropsychiatric patients contributes to their disease.10 For example, heritability for bipolar disorder (BPD) and schizophrenia (SCZ) is estimated to be between 80 and 90%.11, 12 This degree of heritability is higher than that associated with breast cancer (30%),13 type II diabetes (50–70%)14 and hypertension (40–60%).15 Moreover, genome-wide association studies show that psychiatric diseases are generally polygenic, with several genetic variants contributing a fraction of the overall risk and phenotype.16 Further complicating matters, many of these genetic variants exist in non-coding regions with unknown functions.16 Notable exceptions include Rett and Fragile X Syndromes, which arise from defined monogenetic risk factors.17
