In biology, a mutation is a change in the nucleotide sequence of the genome of an organism, virus, or other genetic elements. Mutations may or may not produce discernible changes in the observable characteristics (phenotype) of an organism. Mutations play an important role in both normal and abnormal biological processes such as: evolution, cancer, and the development of the immune system, including junction diversity.

Mutations can result in several different types of sequence changes; such changes could have no effect, alter the product of a gene, or prevent it from functioning properly or completely.