The RNA-guided endonuclease CRISPR-Cas9 is being widely adopted as the genome-editing tool of choice. However, the physical size of the commonly used Cas9 from Streptococcus pyogenes (SpCas9) poses problems for applications that use the adeno-associated virus (AAV) as a delivery vehicle. Feng Zhang and colleagues now characterize six smaller Cas9 orthologues. Focusing on Cas9 fromStaphylococcus aureus, they packaged it and its single guide RNA expression cassette into a single AAV vector and targeted the cholesterol regulatory gene Pcsk9 in the mouse liver. Greater than 40%gene modification was observed within a week of injection, accompanied by significant reductions in serum Pcsk9 and total cholesterol levels.