Copy number variation refers to a molecular phenomenon that involves a repetition of genome sequences. The number of repeats is not standard. It can vary between individuals of the same species. There are two main types of copy number variations based on the size of the repeat: 

• Short repeats, which typically involve dinucleotide and trinucleotide repeats
• Long repeats,which involve repeats of entire genes 

The boundaries between the two types are not clear and often overlap.