Figure 3 Endogenous repair of double-strand DNA breaks by non-homologous end joining (NHEJ) or homology directed repair (HDR) Figure 3 | Endogenous repair of double-strand DNA breaks by non-homologous end joining (NHEJ) or homology directed repair (HDR). Double-strand DNA breaks (DSB) generated by Cas9 in the absence of a donor template are repaired by NHEJ, which is frequently inaccurate, resulting in the insertion of small deletions or insertions. Inclusion of a donor template can result in HDR, leading to the introduction of desired mutations or modifications through homologous recombination. Gibson, G. J. & Yang, M. (2017) What rheumatologists need to know about CRISPR/Cas9 Nat. Rev. Rheumatol. doi:10.1038/nrrheum.2017.6