What are optimal lengths for HDR template homology arms?

It has been demonstrated that there is an exponential relationship between homology arm length and knockin efficiency, with ssDNA templates including homology arms 350–700 nt in length providing optimal performance (Li et al. 2017). In our own studies involving hiPSCs, optimal performance was observed with 350-nt homology arms, and the use of longer arms did not substantially increase knockin efficiencies.
It is important to note that using longer homology arms will increase the molecular weight of your HDR template, and since the amounts of HDR template used for electroporation are usually measured in µg, longer homology arms will correspond with fewer mols (copies) of template being introduced in the target cells, possibly impacting the overall percentage of HDR.
References
Li, H et al., Design and specificity of long ssDNA donors for CRISPR-based knock-in. bioRxiv doi: https://doi.org/10.1101/178905 (2017).

Gene editing products