Abstract: The present disclosure provides a non-naturally occurring CRISPR-Cas system comprising: a Cas9 effector protein capable of generating cohesive ends (stiCas9), and a guide polynucleotide that forms a complex with the stiCas9 and comprising a guide sequence, wherein the guide sequence hybridizes with a target sequence in a eukaryotic cell but does not hybridize to a sequence in a bacterial cell, and wherein the complex does not occur in nature. The present disclosure also provides a method of introducing a sequence of interest into a chromosome of a cell. Finally, the present disclosure provides for a method of modifying one or more nucleotides using seamless mutagenesis.