Copy-number variation (CNV)

A copy-number variation is a gain or loss of a DNA segment relative to the usual number of copies. It can affect part of a gene, one or more whole genes, or much larger genomic regions. CNVs may be inherited, arise in an egg or sperm, or occur after conception in only some cells. Their effects range from harmless variation to altered gene dosage and disease risk, depending on size, location and the genes involved.