Before analyzing target genome structural aberration and copy number variation (CNV) with ESP, the target genome is usually amplified and conserved with artificial chromosome construction. The classic strategy to construct an artificial chromosome is bacterial artificial chromosome (BAC). Basically, the target chromosome is randomly digested and inserted into plasmids which are transformed and cloned in bacteria. The size of fragments inserted is 150–350 kb. Another commonly used artificial chromosome is fosmid. The difference between BAC and fosmids is the size of the DNA inserted. Fosmids can only hold 40 kb DNA fragments, which allows a more accurate breakpoint determination.