Thermostable Cas9 nucleases. The present invention relates to the field of genetic engineering and more particularly to nucleic acid editing and genome modification. The present invention provides an isolated Cas protein or polypeptide fragment thereof having an amino acid sequence of SEQ ID NO: 1 or a sequence of at least 77% identity therewith. The Cas protein or polypeptide is capable of binding, cleaving, marking or modifying a double stranded target polynucleotide at a temperature in the range 20° C. and 100° C. inclusive. The invention further provides isolated nucleic acid molecules encoding said Cas9 nucleases, expression vectors and host cells. The invention also provides PAM sequences recognized by the Cas protein or polypeptide, The Cas9 nucleases disclosed herein provide novel tools for genetic engineering in general, in particular at elevated temperatures.