The study introduces the use of *Staphylococcus aureus* Cas9 (SaCas9) for in vivo genome editing, demonstrating its efficiency and specificity. SaCas9 is a smaller version of the commonly used *Streptococcus pyogenes* Cas9 (SpCas9), which is packaged into a single AAV vector along with its sgRNA. In vitro, SaCas9 was shown to be as efficient as SpCas9 in cleaving targets 3 bp upstream of the PAM. In vivo, SaCas9 was used to target the *Pcsk9* gene in mouse liver, leading to a >40% gene modification rate and significant reductions in serum Pcsk9 and total cholesterol levels. The authors also developed BLESS, an unbiased method to assess the genome-wide targeting specificity of Cas9, which identified additional off-target sites not previously predicted by sequence similarity. The study highlights the potential of SaCas9 for efficient and specific in vivo genome editing.The study introduces the use of *Staphylococcus aureus* Cas9 (SaCas9) for in vivo genome editing, demonstrating its efficiency and specificity. SaCas9 is a smaller version of the commonly used *Streptococcus pyogenes* Cas9 (SpCas9), which is packaged into a single AAV vector along with its sgRNA. In vitro, SaCas9 was shown to be as efficient as SpCas9 in cleaving targets 3 bp upstream of the PAM. In vivo, SaCas9 was used to target the *Pcsk9* gene in mouse liver, leading to a >40% gene modification rate and significant reductions in serum Pcsk9 and total cholesterol levels. The authors also developed BLESS, an unbiased method to assess the genome-wide targeting specificity of Cas9, which identified additional off-target sites not previously predicted by sequence similarity. The study highlights the potential of SaCas9 for efficient and specific in vivo genome editing.