DNA fragmentation index (DFI) is a measure of the percentage of sperm in a semen sample with fragmented DNA. A higher DFI indicates that a larger percentage of a person’s sperm contains genetic damage. While experts disagree on specific cut-off values, generally speaking, a DFI over 30-50% is considered high and may impact fertility. The DFI can be measured by the Comet assay or by the TUNEL assay. The intentional fragmentation of DNA is often necessary prior to library construction or subcloning for DNA sequences, and a variety of methods involving the mechanical breakage of DNA have been employed where DNA is fragmented by laboratory personnel.