A candidate gene typically is chosen because it is suspected to play a role in the disorder, either because researchers have some information about the gene’s function that might be related to the disease or because the gene lies in a DNA region that has already been linked to the disorder through linkage studies. In the future, researchers hope to be able to conduct genomewide association studies in which SNPs spaced across the entire genome are tested to identify potential associations with a disease, rather than having to focus on candidate regions or genes. Such an approach, however, is likely to require 50,000 to 300,000 SNPs to test all DNA regions for associations with a disease or trait. The identification of large numbers of SNPs spaced throughout the genome is already under way. In addition, researchers are currently developing new methods to make it feasible for genetic laboratories to determine a person’s DNA sequences (i.e., genotypes) for the large number of markers that will be required in genomewide association studies. Finally, new approaches to the statistical analysis of the vast
