I am concerned about the possible off target effects of long term Cas9 expression. Can you direct me to any research being done on this? Thanks

Answer

You can express Cas9 transiently and avoid integration into the genome. Published work with Cas9 nickase and dimeric RNA guided Fok1 nucleases have shown relatively more specificity (http://www.nature.com/nbt/journal/v32/n6/full/nbt.2908.html; http://zlab.mit.edu/assets/reprints/Ran_FA_Cell_2013.pdf). More recently it has been shown that direct transfection of Cas9 protein works “almost immediately after delivery and are degraded rapidly in cells, reducing off-target effects” (http://genome.cshlp.org/content/early/2014/04/02/gr.171322.113).