Single-nucleotide polymorphism (SNP) is a variation at a single position in a DNA sequence among individuals. In other words, it's a change in a single nucleotide—the building blocks of DNA (adenine [A], cytosine [C], guanine [G], or thymine [T])—that can occur in the genome. SNPs can manifest in several ways, typically as a substitution of one nucleotide for another.
