Fecha de publicación: -- Fuente: WIPO "tomato" Disclosed herein are novel polypeptides having nuclease activity. The polypeptides can effect modifications at defined locations within target nucleic acids, e.g., DNA and RNA. Also disclosed herein modified guide RNAs. The polypeptides and guide RNAs are useful in compositions, systems and methods, to make site specific alterations of target nucleic acid sequences.