Karvelis, T. et. a. (2017) Current Opinions in Microbiology 37:88-94. https://www.ncbi.nlm.nih.gov/pubmed/28645099
While the CRISPR/Cas9 system is a significant improvement over previous gene editing technology, limitations remain. A significant constraint is the requirement for the PAM sequence. By identifying Cas9 variants from additional species and in vitro evolution of identified variants, scientists will be able to diversify the number of sequences targeted.