The CRISPR-Cas9 system has proven to be a powerful tool for genome editing, allowing for the precise modification of specific DNA sequences within a cell. Many efforts are currently underway to use the CRISPR-Cas9 system for the therapeutic correction of human genetic diseases. The best characterized CRISPR-associated nucleases are the Cas9 proteins from Streptococcus pyogenes (S. pyogenes) and Staphylococcus aureus (S. aureus).