Chen, S. et al. Cell Rep. 27, 3780–3789 (2019)
The mouse has proved its mettle as a model organism to test gene function in vivo and to model disease. But, despite its popularity, it is still not trivial to create a transgenic mouse with large insertions or to swap an endogenous gene with a transgene. To streamline this process, Chen et al. have combined the CRISPR system with adeno-associated virus (AAV)-mediated delivery of donor DNA in a method they call CRISPR–READI. Mouse embryos are transduced with an AAV harboring the donor DNA; the Cas9–sgRNA complex, targeting the insertion site, is then delivered into the mouse embryos via electroporation. The treated embryos are implanted back into a mouse and the offspring is tested for editing. The authors see up to 48% homology-directed recombination (HDR). They insert transgenes of over 3 kb, only limited by the AAV packaging capacity of 4.9 kb. It will be interesting to explore the ability of CRISPR–READI to generate multiplexed gene insertions.