任何有潛在性的會導致一個或更多基因的功能變化的DNA的改變。突變可以是DNA的單個堿基變化（點突變）或一個基因中的一對堿基對丟失（缺失突變），或染色體的一段移位從而影響很多基因。突變可以是由輻射、化學作用、外部病原體（如：病毒）或細胞分裂時基因組復制錯誤造成。突變會影響基因調節和表達或造成蛋白質本身變化從而產生無功能蛋白或異常蛋白。有些DNA的變化是自然發生的不會產生有害影響；這些在一個群體中的變化被稱為多態性。

Any alteration to DNA that can potentially result in a change in the function of one or more genes. Mutations can be a change in a single base of DNA (point mutation) or a loss of base pairs (deletion) affecting a single gene, or a movement of chromosomal regions (translocation) affecting many genes. Mutations can be induced by radiation, chemical treatment, foreign pathogens (e.g. viruses), or due to errors that occur during replication of the genome each time a cell divides. Mutations can affect gene regulation and expression or can cause a change in the protein itself that results in a non-functional protein (e.g. the CFTR protein defective in cystic fibrosis) or one with abnormal activity (e.g. the p53 cancer-causing protein). Some changes in DNA occur naturally and lead to no harmful effects; these changes in a population are called polymorphisms.