Kleinstiver, B.P. et al, Nature (2015) 523:481-485. http://www.ncbi.nlm.nih.gov/pubmed/26098369
While the construction of unique guide RNAs theoretically allows CRISPR/Cas9 systems to target any unique genomic sequence the range of possible sequences is limited by the need for a specific protospacer adjacent motif (PAM). For instance, the widely adopted Cas9 endonuclease requires an NGG PAM to directly follow the guide RNA sequence. To overcome this limitation Kleinstiver et al engineered Cas9 variants to recognize novel PAM sequences using a directed evolution approach thus expanding the range of sequences CRISPR/Cas9 systems can be used to edit.