> Homologous Recombination

Homologous Recombination (HR) is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. It is a natural safety mechanism used by cells to accurately repair harmful breaks that occur on both strands of DNA, known as double-strand breaks (DSBs), induced by UV damage, chemical agents, etc. This recombination mechanism is well preserved throughout evolution and across species, and is an essential factor in cell survival.

HR has made a significant contribution to genome engineering, as it is the basis of gene targeting (targeted integration and gene modification) and remains the safest and cleanest way to modify a genome. Its most striking characteristic is to enable the replacement of one damaged DNA sequence by a homologous one with a high level of precision.

> Non Homologous End-Joining

Non Homologous End-Joining (NHEJ) is a cell natural process that repairs DSBs in DNA. NHEJ is referred to as "non-homologous" because the break ends are directly re-ligated without the need for a homologous template, in contrast to HR.

Due to the error-prone nature of NHEJ, a proportion of DSBs will be misrepaired by the addition and/or deletion of nucleotides. These mutations within the genomic sequence occur at the site of the DSB, resulting in the loss of gene function and therefore achieving gene knock-out.