Having decided on the study design and study samples, the next step is to select appropriate candidate genes. In general, a gene is selected with some a priori relationship with the disorder, based on its localization (i.e. the gene is located in a chromosomal region with a significant linkage), or proposed function in the pathogenesis of the disorder (e.g. the gene belongs to the dopamine or serotonin pathway in association studies for schizophrenia pathogenesis). Genome-wide association (GWA) studies can now also be performed, whereby large numbers of DNA polymorphisms are analyzed in one experiment.
