Most common CNS disorders are highly polygenic in nature; that is, they are controlled by complex interactions between numerous gene products. As such, these conditions do not exhibit the single gene defect basis that is so attractive for the development of highly-specific drugs largely free of major undesirable side-effects ("the magic bullet"). Second, the exact nature of the interactions that occur between the numerous gene products typically involved in CNS disorders remain elusive, and the biological mechanisms underlying mental illnesses are poorly understood.