SNPs can be of particular benefit in studies of complex disorders for which many potential candidate genes exist. For example, linkage mapping studies have suggested several genomic areas that may contain susceptibility genes for alcoholism. Each of these areas, however, is so large that it may contain dozens or hundreds of genes depending on the size and gene density of each region.4 Because it would be prohibitively difficult to sequence all these genes, publicly available SNP data are a great resource for candidate gene and association studies. For example, researchers recently analyzed several SNPs in the DNA region containing a candidate gene for Alzheimer’s disease (AD) and demonstrated that two SNPs closely flanking that gene indeed showed strong association with AD (Martin et al. 2000). (For more information on this candidate gene for AD, see the section “Examples of the Candidate Gene Approach in Humans.”)
