Wang, Y., et. al. Scientific Reports (2016) 6:31145 http://www.ncbi.nlm.nih.gov/pubmed/27503169
Two of the challenges facing CRISPR/Cas9 gene editing are off-target effects and Cas9 toxicity due to prolonged Cas9/gRNA expression. In order to prevent these adverse outcomes Wang et al. have developed a CRISPR/Cas9 system that will spontaneously eliminate itself co-transforming with a deletion construct that knocks out Cas9 expression during induced crossing over events.