The number of copies of a particular region of the genome occurring in a cell or an individual. Deletions or duplications affecting segments of DNA greater than about 1 kilobase cause copy number variation. These variations can affect up to 20% of the genome in humans. Some occur in stretches of DNA between genes where they may have little or no impact, whereas others can affect gene function and cause disease. For example, a three-nucleotide sequence is often found repeated numerous times in tandem, forming a series of trinucleotide repeats. Expansion of these near to or within certain genes results in several human diseases, such as Huntington disease and fragile X syndrome. Compare single nucleotide polymorphism.