A copy number variation (CNV ) is a segment of DNA in which copy-number differences have been found by comparison of two or more genomes. The segment may range from one kilobase to several megabases in size.Humans (being diploid) ordinarily have two copies of each autosomal region, one per chromosome. This may vary for particular genetic regions due to deletion or duplication.CNVs may either be inherited or caused by de novo mutation. A recently proposed mechanism for the cause of some CNVs is fork stalling and template switching, a replication misstep.