hello Jana, these questions cannot be answered in general as the efficiency of your CRISPR/Cas reagents is strongly depended on your gRNA design, on the expression level etc. Furthermore the homologous recombination efficacy itself is also strongly depended on the genomic region (local chromatin structure e.g.). clearly longer arm can help to increase HR, but short arms can be sufficient. E.g. recombination is very efficient using single stranded oligonucleotides with only 40 -50 bp homology on each site, assuming a point mutation in the center. 

Hi,

I agree the length of your homology arms is dictated by the efficiency of your CRISPR/Cas9 gRNA. Have you considered PiTCH donors for MMEJ as an alternative, also efficiency of using short homology arms can be increased using asymmetrical and single stranded donors

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Hi Jana, 

I don't understand why you need such a short arm. Even if there is a promoter, you could still use that region as a homology arm, couldn't you? Or is it because it will be a GC-high region difficult to amplify? 

In any case, I am sure it will work. I have gone to ~400bp in one arm (because there was a highly repetitive region a little further), and it worked. It may be less efficient, but you will have the PUTK selection to select for the correctly targeted cells. 

Another strategy to increase specificity and not having to rely on HR would be to use a double nickase approach, or the new eSpCas9... Just a thought. 

Good luck!