Introduction of a DSB into the genome allows many different types of genetic manipulation. Usually, this DNA damage is repaired by one of two major pathways, non-homologous end joining (NHEJ) that performs non-templated ligation of the free DNA ends, and homology directed repair (HDR) that utilises homologous DNA to direct a precise repair (Bibikova et al. 2003; Shrivastav et al. 2008). Both of these can be exploited for making site-specific genetic mutations. Repair through NHEJ or related pathways such as microhomology mediated end joining (MMEJ) can result in insertions or deletions (indels) of several nucleotides at the DSB that can be used for instance to introduce a frameshift in protein coding sequence, resulting in a null allele. If two DSBs are made simultaneously, the NHEJ machinery can also ligate the wrong termini together, resulting in deletions, inversions or translocations (Torres et al. 2014; Xiao et al. 2013; Park et al. 2016). In the context of more defined changes to the DNA such as introduction of SNPs, HDR pathways can be utilised by providing an excess of a desired sequence, resulting
