Genetically modified mice can be generated using a variety of methods. Depending on the method used, the breeding strategy must be modified not only for the initial founding generation of mouse, but to establish individual mouse lines as well. Transgenic founder mice are each unique and must be outcrossed to initially establish the line. Mice that have been targeted using embryonic stem cells will need to be tested for germ line transmission of the modified gene prior to establishing a line. Mice that have been targeted using CRISPR/Cas9 gene-editing endonucleases will often be mosaic, and thus, also require testing for germ line transmission of the mutation of interest.