As far as we know, the major mental illnesses are not single-gene disorders. Unlike certain medical and neurological disorders (for example, Huntington’s disease), psychiatric disorders cannot be attributed to any one gene. While multiple genes have been linked to psychiatric disorders (for example, neureulin-1, catechol O-methyl-transferase, and dysbindin genes for schizophrenia), these genes are best understood as risk factors for the disease rather than as definite causes. Not all people with the gene will have the disorder and not all people with the disorder will have the gene. 8 Therefore, geneticists now believe that most psychiatric disorders are related to a whole series of genes, only some of which are currently known. Any one of these vulnerability genes raises the risk of the disorder, but not by a large amount. The greater the number of vulnerability genes that any given person has, the greater the risk of developing the disorder.