What are single nucleotide polymorphisms (SNPs)?

Single nucleotide polymorphisms, frequently called SNPs (pronounced “snips”), are the most common type of genetic variation among people. Each SNP represents a difference in a single DNA building block, called a nucleotide. For example, a SNP may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain stretch of DNA.

SNPs occur normally throughout a person’s DNA. They occur once in every 300 nucleotides on average, which means there are roughly 10 million SNPs in the human genome. Most commonly, these variations are found in the DNA between genes. They can act as biological markers, helping scientists locate genes that are associated with disease. When SNPs occur within a gene or in a regulatory region near a gene, they may play a more direct role in disease by affecting the gene’s function.

Most SNPs have no effect on health or development. Some of these genetic differences, however, have proven to be very important in the study of human health. Researchers have found SNPs that may help predict an individual’s response to certain drugs, susceptibility to environmental factors such as toxins, and risk of developing particular diseases. SNPs can also be used to track the inheritance of disease genes within families. Future studies will work to identify SNPs associated with complex diseases such as heart disease, diabetes, and cancer.

For more information about SNPs:

An audio definition of SNPs is available from the National Human Genome Research Institute’s Talking Glossary of Genetic Terms.

The NCBI Science Primer offers a detailed description of SNPs in the chapter titled SNPs: Variations on a Theme.

The U.S. Department of Energy Office of Science provides additional information in its SNP Fact Sheet.

A detailed overview of SNPs and their association with cancer risk can be found in the National Cancer Institute’s Understanding Cancer Series: Genetic Variation (SNPs).

For people interested in more technical data, several databases of known SNPs are available:

• NCBI database of single nucleotide polymorphisms (dbSNP)

• Database of Japanese single nucleotide polymorphisms (JSNP)

Next: What are genome-wide association studies?