Paper: #CRISPR optimization in mouse ESCs, with 50 vs 200 bp arm comparison for HDR. Longer arms better.

 This was just out in PLoS ONE: 
Optimization of Genome Engineering Approaches with the CRISPR/Cas9 System, by Li et al.   The final figure shows a direct comparison between 50 and 200 bp homology arms for GFP knock-in via CRISPR-mediated HDR;  no big surprise, but clearly 200 bp had greater efficiency to 50 bp, by about an order of magnitude.   This is useful info for anyone who is designing "large" knock-in cassettes and wondering if it's important to put long arms on the cassettes; although ~60 bases is commonly used for ssDNAs in HDR (i.e. for peptide tag or loxP insertion), it's important to lengthen them for longer cassettes.    I suggest ~1 kb.  (my gut feeling as of today).