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What are single nucleotide polymorphisms (SNPs)?
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Genomicresearch

What are single nucleotide polymorphisms (SNPs)?

Single nucleotide polymorphisms, or SNPs, are the most common type of genetic variation among people, representing a difference in a single DNA building block.

Overview

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.

Prevalence and Classification

SNPs occur normally throughout a person’s DNA. They occur almost once in every 1,000 nucleotides on average, which means there are roughly 4 to 5 million SNPs in a person's genome. To be classified as a SNP, a variant must be found in at least 1 percent of the population. Scientists have identified more than 600 million SNPs in populations around the world.

Role in Health and Disease

Most commonly, SNPs 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.

While most SNPs have no effect on health or development, some are important in the study of human health:

  • They help predict an individual’s response to certain drugs.
  • They indicate susceptibility to environmental factors, such as toxins.
  • They help assess the risk of developing diseases.
  • They can be used to track the inheritance of disease-associated genetic variants within families.

Research is currently ongoing to identify SNPs associated with complex diseases such as heart disease, diabetes, and cancer.

Frequently Asked Questions

SNPs within coding regions can change the amino acid sequence of a protein, potentially altering its function. SNPs in regulatory or non‑coding regions can influence how much of a gene is expressed or when it is turned on, which can also affect disease risk or drug response. Most SNPs in intergenic areas are benign, but some serve as markers for nearby functional variants.
Yes, certain SNPs in drug‑metabolizing enzymes or drug targets can alter drug efficacy or increase the risk of side effects. For example, a SNP in the CYP2C19 gene can reduce clopidogrel activation, leading to less antiplatelet effect. Knowing your SNP status can guide dose adjustments or alternative therapies.
SNPs follow Mendelian inheritance: each parent contributes one allele for each SNP. If a SNP is present in one parent, there is a 50 % chance it will be passed to each child. However, many SNPs are common in the population, so the presence of a particular SNP does not necessarily indicate a disease risk on its own.
You can order a direct‑to‑consumer genetic test from a reputable company that reports SNPs linked to health traits. Alternatively, if you have a medical condition, ask your clinician about a targeted genetic panel that includes relevant SNPs. The results will be delivered in a report that lists the SNPs found and, when available, the associated risk or trait information.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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