Not all genes, however, have an easily identifiable common functional variant that can be exploited in association studies, and in many cases researchers have identified only changes in individual nucleotides (i.e., single nucleotide polymorphisms [SNPs]) that have no known functional significance. Nevertheless, SNPs can be potentially useful in narrowing a linkage region. In addition, they may show a statistically significant association with a disease susceptibility gene if they are located within or near that gene by virtue of linkage disequilibrium (see the sidebar for a description of this phenomenon).
