Association studies assess polymorphisms— genetic differences between individuals at specific locations of the genome. These polymorphisms, which can be located within a gene (where they would give rise to different gene variants, or alleles) but also outside of genes, are used to identify regions of the genome that are located at or near a disease susceptibility gene. Most recently, candidate gene association studies have been conducted using single-nucleotide polymorphisms (SNPs). Nucleotides are the building blocks of the DNA. Accordingly, SNPs are DNA segments that differ in only a single building block and can therefore be considered the smallest unit of human genetic variation. The identification of SNPs became possible only after the Human Genome Project, which deciphered the entire human DNA sequence, had been completed, and additional technological advances since then have facilitated the study of SNPs in large samples. The main advantage of using SNPs in candidate gene studies is that they allow researchers to locate disease susceptibility genes with greater precision than before. Although earlier genetic studies had to rely on a relatively small number of polymorphic markers, the density of markers has dramatically increased thanks to the SNPs.
