A copy number variation (CNV) is when the number of copies of a particular gene varies from one individual to the next. Following the completion of the Human Genome Project, it became apparent that the genome experiences gains and losses of genetic material. The extent to which copy number variation contributes to human disease is not yet known. It has long been recognized that some cancers are associated with elevated copy numbers of particular genes.


Copy number variation is a type of structural variation where you have a stretch of DNA, which is duplicated in some people, and sometimes even triplicated or quadruplicated. And so when you look at that chromosomal region, you will see a variation in the number of copies in normal people. Sometimes those copy number variants include genes, maybe several genes, which may mean that this person has four copies of that gene instead of the usual two, and somebody else has three, and somebody else has five. Interesting, we didn't really expect to see so much of that. It's now turning out to be pretty common, and in some instances, if those genes are involved in functions that are sensitive to the dosage, you might then see a consequence in terms of a disease risk.

- Francis S. Collins, M.D., Ph.D.