SNP stands for Single Nucleotide Polymorphism, which is 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, and they occur almost once in every 1,000 nucleotides on average, which means there are roughly 4 to 5 million SNPs in a persons genome. To be classified as a SNP, a variant is found in at least 1 percent of the population.

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. Most SNPs have no effect on health or development, but some of these genetic differences have proven to be very important in the study of human health. SNPs help predict an individual’s response to certain drugs, susceptibility to environmental factors such as toxins, and risk of developing diseases.

Scientists study if and how SNPs in a genome influence health, disease, drug response, and other traits. Reliable SNPs could serve as predictive markers that inform our decisions about numerous aspects of medical care, including specific diseases, effectiveness of various drugs, and adverse reactions to specific drugs. This pharmacogenetic approach could save time, money, and discomfort for millions of patients through accurate diagnoses and matching patients with appropriate medicines.