Recombinant Cas9 protein from Streptococcus pyogenes (~160 KD) is a ready-to-use reagent for genome engineering experiments. When combined with target-specific guide RNAs wild type Streptococcus pyogenes Cas9 protein will act as a targeted nuclease suitable for transfection of cell cultures and for the accelerated development of genetically-modified animals via one-cell embryo injection.