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).