People have two copies of most genes, one copy inherited from each parent. In some cases, however, the number of copies varies—meaning that a person can have one, three, or more copies of particular genes. Less commonly, both copies of a gene may be missing. These types of genetic difference are known as copy number variations (CNV).

Copy number variation results from insertions, deletions, and duplications of  large segments of DNA that are at least one thousand nucleotides (also called 1 kilobase or 1kb) in length. These segments are often big enough to include whole genes. Variation in gene copy number can influence the activity of genes and the functioning of proteins made from them, which may affect body processes.

Copy number variation accounts for a significant amount of genetic difference between people. More than 10 percent of the human genome appears to contain differences in gene copy number. While much of this variation does not affect health or development, some differences influence a person’s risk of disease, particularly some types of cancer, or response to certain drugs.