Due to the high heritability of schizophrenia1, there have been many efforts to discover the causative genetic factors, and candidate gene studies have been a major approach. For example, the SZGene database 2 (obtained 11/2009) listed 1406 candidate gene papers investigating over 700 genes. In these studies, one or more genetic markers in genes hypothesized to be involved in the etiology of schizophrenia were genotyped in cases with schizophrenia and controls. Prior to the advances brought about by the Human Genome Project3 and the International HapMap Project 4, it was difficult and expensive to genotype a comprehensive list of genetic variants in a genomic region. Investigators thus tended to genotype a few genetic markers in a candidate gene selected based on prevailing theories of the etiology of schizophrenia (e.g., antipsychotic pharmacology) or positional candidate genes from linkage or cytogenetic studies.
