Because of the phenotypic complexity of psychiatric illnesses, the search for pleiotropic genes may help clarify psychiatric diagnoses and help understand biologic causes of mental illness. For example, suspicion of shared genetic susceptibilities with bipolar disorder and illegal substance dependence has been the subject of recent research. A bipolar GWAS data set and an illegal substance dependence GWAS data set were compared for overlap of top signals, and evidence was found of shared associations. 63 These studies suggest that advances may be made by understanding genetic pleiotropy and interconnections between addiction and other major psychiatric illnesses.
