A single-nucleotide polymorphism (SNP) is a difference at a single spot in a DNA strand called a nucleotide. Nucleotides are the building blocks of messages encoded by DNA.

Each strand of DNA has nucleotides consisting of four types of bases: adenine (A), cytosine (C), guanine (G), and thymine (T). Each strand pairs together with a second DNA strand to make up a full DNA double helix. The individual bases on one strand each pair with a corresponding base on the other strand; this pairing is called a base pair. Each type of base can pair with only one other type of base: T binds with A and G binds with C. However, in SNPs, a single base pair will be different from what’s found in most people, leading to the term “polymorphism”. And since it’s only at one nucleotide, it’s called a “single-nucleotide polymorphism”. Because of this change, the DNA may read differently and lead to changes in activities of the body.