Compositions and methods are provided for genome modification of a target sequence in the genome of a cell. The methods and compositions employ a guide polynucleotide/Cas endonuclease system to provide an effective system for modifying or altering target sites within the genome of a cell or organism. Once a genomic target site is identified, a variety of methods can be employed to further modify the target sites such that they contain a variety of polynucleotides of interest. Compositions and methods are also provided for editing a nucleotide sequence in the genome of a cell. Breeding methods and methods for selecting plants utilizing a two component RNA polynucleotide and Cas endonuclease system are also disclosed.
