single nucleotide polymorphism (SING-gul NOO-klee-oh-tide PAH-lee-MOR-fih-zum)
The most common type of change in DNA (molecules inside cells that carry genetic information). Single nucleotide polymorphisms occur when a single nucleotide (building block of DNA) is replaced with another. These changes may cause disease, and may affect how a person reacts to bacteria, viruses, drugs, and other substances. Also called SNP.